Rename Screens to Activities and restructure files (#44)

## Summary

* This PR drastically reshapes the structure of the codebase, moving
from the concept of "Screens" to "Activities", restructing the files and
setting up the concept of subactivities.
* This should help with keep the main file clean and containing all
functional logic in the relevant activity.
* CrossPointState is now also a global singleton which should help with
accessing it from within activities.

## Additional Context

* This is probably going to be a bit disruptive for people with open
PRs, sorry 😞

src/activities/reader/FileSelectionActivity.cpp

@@ -0,0 +1,147 @@
+#include "FileSelectionActivity.h"
+
+#include <GfxRenderer.h>
+#include <SD.h>
+
+#include "config.h"
+
+void sortFileList(std::vector<std::string>& strs) {
+  std::sort(begin(strs), end(strs), [](const std::string& str1, const std::string& str2) {
+    if (str1.back() == '/' && str2.back() != '/') return true;
+    if (str1.back() != '/' && str2.back() == '/') return false;
+    return lexicographical_compare(
+        begin(str1), end(str1), begin(str2), end(str2),
+        [](const char& char1, const char& char2) { return tolower(char1) < tolower(char2); });
+  });
+}
+
+void FileSelectionActivity::taskTrampoline(void* param) {
+  auto* self = static_cast<FileSelectionActivity*>(param);
+  self->displayTaskLoop();
+}
+
+void FileSelectionActivity::loadFiles() {
+  files.clear();
+  selectorIndex = 0;
+  auto root = SD.open(basepath.c_str());
+  for (File file = root.openNextFile(); file; file = root.openNextFile()) {
+    auto filename = std::string(file.name());
+    if (filename[0] == '.') {
+      file.close();
+      continue;
+    }
+
+    if (file.isDirectory()) {
+      files.emplace_back(filename + "/");
+    } else if (filename.substr(filename.length() - 5) == ".epub") {
+      files.emplace_back(filename);
+    }
+    file.close();
+  }
+  root.close();
+  sortFileList(files);
+}
+
+void FileSelectionActivity::onEnter() {
+  renderingMutex = xSemaphoreCreateMutex();
+
+  basepath = "/";
+  loadFiles();
+  selectorIndex = 0;
+
+  // Trigger first update
+  updateRequired = true;
+
+  xTaskCreate(&FileSelectionActivity::taskTrampoline, "FileSelectionActivityTask",
+              2048,               // Stack size
+              this,               // Parameters
+              1,                  // Priority
+              &displayTaskHandle  // Task handle
+  );
+}
+
+void FileSelectionActivity::onExit() {
+  // Wait until not rendering to delete task to avoid killing mid-instruction to EPD
+  xSemaphoreTake(renderingMutex, portMAX_DELAY);
+  if (displayTaskHandle) {
+    vTaskDelete(displayTaskHandle);
+    displayTaskHandle = nullptr;
+  }
+  vSemaphoreDelete(renderingMutex);
+  renderingMutex = nullptr;
+  files.clear();
+}
+
+void FileSelectionActivity::loop() {
+  const bool prevPressed =
+      inputManager.wasPressed(InputManager::BTN_UP) || inputManager.wasPressed(InputManager::BTN_LEFT);
+  const bool nextPressed =
+      inputManager.wasPressed(InputManager::BTN_DOWN) || inputManager.wasPressed(InputManager::BTN_RIGHT);
+
+  if (inputManager.wasPressed(InputManager::BTN_CONFIRM)) {
+    if (files.empty()) {
+      return;
+    }
+
+    if (basepath.back() != '/') basepath += "/";
+    if (files[selectorIndex].back() == '/') {
+      basepath += files[selectorIndex].substr(0, files[selectorIndex].length() - 1);
+      loadFiles();
+      updateRequired = true;
+    } else {
+      onSelect(basepath + files[selectorIndex]);
+    }
+  } else if (inputManager.wasPressed(InputManager::BTN_BACK)) {
+    if (basepath != "/") {
+      basepath = basepath.substr(0, basepath.rfind('/'));
+      if (basepath.empty()) basepath = "/";
+      loadFiles();
+      updateRequired = true;
+    } else {
+      // At root level, go back home
+      onGoHome();
+    }
+  } else if (prevPressed) {
+    selectorIndex = (selectorIndex + files.size() - 1) % files.size();
+    updateRequired = true;
+  } else if (nextPressed) {
+    selectorIndex = (selectorIndex + 1) % files.size();
+    updateRequired = true;
+  }
+}
+
+void FileSelectionActivity::displayTaskLoop() {
+  while (true) {
+    if (updateRequired) {
+      updateRequired = false;
+      xSemaphoreTake(renderingMutex, portMAX_DELAY);
+      render();
+      xSemaphoreGive(renderingMutex);
+    }
+    vTaskDelay(10 / portTICK_PERIOD_MS);
+  }
+}
+
+void FileSelectionActivity::render() const {
+  renderer.clearScreen();
+
+  const auto pageWidth = GfxRenderer::getScreenWidth();
+  renderer.drawCenteredText(READER_FONT_ID, 10, "CrossPoint Reader", true, BOLD);
+
+  // Help text
+  renderer.drawText(SMALL_FONT_ID, 20, GfxRenderer::getScreenHeight() - 30, "Press BACK for Home");
+
+  if (files.empty()) {
+    renderer.drawText(UI_FONT_ID, 20, 60, "No EPUBs found");
+  } else {
+    // Draw selection
+    renderer.fillRect(0, 60 + selectorIndex * 30 + 2, pageWidth - 1, 30);
+
+    for (size_t i = 0; i < files.size(); i++) {
+      const auto file = files[i];
+      renderer.drawText(UI_FONT_ID, 20, 60 + i * 30, file.c_str(), i != selectorIndex);
+    }
+  }
+
+  renderer.displayBuffer();
+}