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/EpubReaderChapterSelectionActivity.cpp

@@ -0,0 +1,107 @@
+#include "EpubReaderChapterSelectionActivity.h"
+
+#include <GfxRenderer.h>
+#include <SD.h>
+
+#include "config.h"
+
+constexpr int PAGE_ITEMS = 24;
+constexpr int SKIP_PAGE_MS = 700;
+
+void EpubReaderChapterSelectionActivity::taskTrampoline(void* param) {
+  auto* self = static_cast<EpubReaderChapterSelectionActivity*>(param);
+  self->displayTaskLoop();
+}
+
+void EpubReaderChapterSelectionActivity::onEnter() {
+  if (!epub) {
+    return;
+  }
+
+  renderingMutex = xSemaphoreCreateMutex();
+  selectorIndex = currentSpineIndex;
+
+  // Trigger first update
+  updateRequired = true;
+  xTaskCreate(&EpubReaderChapterSelectionActivity::taskTrampoline, "EpubReaderChapterSelectionActivityTask",
+              2048,               // Stack size
+              this,               // Parameters
+              1,                  // Priority
+              &displayTaskHandle  // Task handle
+  );
+}
+
+void EpubReaderChapterSelectionActivity::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;
+}
+
+void EpubReaderChapterSelectionActivity::loop() {
+  const bool prevReleased =
+      inputManager.wasReleased(InputManager::BTN_UP) || inputManager.wasReleased(InputManager::BTN_LEFT);
+  const bool nextReleased =
+      inputManager.wasReleased(InputManager::BTN_DOWN) || inputManager.wasReleased(InputManager::BTN_RIGHT);
+
+  const bool skipPage = inputManager.getHeldTime() > SKIP_PAGE_MS;
+
+  if (inputManager.wasPressed(InputManager::BTN_CONFIRM)) {
+    onSelectSpineIndex(selectorIndex);
+  } else if (inputManager.wasPressed(InputManager::BTN_BACK)) {
+    onGoBack();
+  } else if (prevReleased) {
+    if (skipPage) {
+      selectorIndex =
+          ((selectorIndex / PAGE_ITEMS - 1) * PAGE_ITEMS + epub->getSpineItemsCount()) % epub->getSpineItemsCount();
+    } else {
+      selectorIndex = (selectorIndex + epub->getSpineItemsCount() - 1) % epub->getSpineItemsCount();
+    }
+    updateRequired = true;
+  } else if (nextReleased) {
+    if (skipPage) {
+      selectorIndex = ((selectorIndex / PAGE_ITEMS + 1) * PAGE_ITEMS) % epub->getSpineItemsCount();
+    } else {
+      selectorIndex = (selectorIndex + 1) % epub->getSpineItemsCount();
+    }
+    updateRequired = true;
+  }
+}
+
+void EpubReaderChapterSelectionActivity::displayTaskLoop() {
+  while (true) {
+    if (updateRequired) {
+      updateRequired = false;
+      xSemaphoreTake(renderingMutex, portMAX_DELAY);
+      renderScreen();
+      xSemaphoreGive(renderingMutex);
+    }
+    vTaskDelay(10 / portTICK_PERIOD_MS);
+  }
+}
+
+void EpubReaderChapterSelectionActivity::renderScreen() {
+  renderer.clearScreen();
+
+  const auto pageWidth = renderer.getScreenWidth();
+  renderer.drawCenteredText(READER_FONT_ID, 10, "Select Chapter", true, BOLD);
+
+  const auto pageStartIndex = selectorIndex / PAGE_ITEMS * PAGE_ITEMS;
+  renderer.fillRect(0, 60 + (selectorIndex % PAGE_ITEMS) * 30 + 2, pageWidth - 1, 30);
+  for (int i = pageStartIndex; i < epub->getSpineItemsCount() && i < pageStartIndex + PAGE_ITEMS; i++) {
+    const int tocIndex = epub->getTocIndexForSpineIndex(i);
+    if (tocIndex == -1) {
+      renderer.drawText(UI_FONT_ID, 20, 60 + (i % PAGE_ITEMS) * 30, "Unnamed", i != selectorIndex);
+    } else {
+      auto item = epub->getTocItem(tocIndex);
+      renderer.drawText(UI_FONT_ID, 20 + (item.level - 1) * 15, 60 + (i % PAGE_ITEMS) * 30, item.title.c_str(),
+                        i != selectorIndex);
+    }
+  }
+
+  renderer.displayBuffer();
+}