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/home/HomeActivity.cpp

@@ -0,0 +1,103 @@
+#include "HomeActivity.h"
+
+#include <GfxRenderer.h>
+#include <SD.h>
+
+#include "config.h"
+
+namespace {
+constexpr int menuItemCount = 2;
+}
+
+void HomeActivity::taskTrampoline(void* param) {
+  auto* self = static_cast<HomeActivity*>(param);
+  self->displayTaskLoop();
+}
+
+void HomeActivity::onEnter() {
+  renderingMutex = xSemaphoreCreateMutex();
+
+  selectorIndex = 0;
+
+  // Trigger first update
+  updateRequired = true;
+
+  xTaskCreate(&HomeActivity::taskTrampoline, "HomeActivityTask",
+              2048,               // Stack size
+              this,               // Parameters
+              1,                  // Priority
+              &displayTaskHandle  // Task handle
+  );
+}
+
+void HomeActivity::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 HomeActivity::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 (selectorIndex == 0) {
+      onReaderOpen();
+    } else if (selectorIndex == 1) {
+      onSettingsOpen();
+    }
+  } else if (prevPressed) {
+    selectorIndex = (selectorIndex + menuItemCount - 1) % menuItemCount;
+    updateRequired = true;
+  } else if (nextPressed) {
+    selectorIndex = (selectorIndex + 1) % menuItemCount;
+    updateRequired = true;
+  }
+}
+
+void HomeActivity::displayTaskLoop() {
+  while (true) {
+    if (updateRequired) {
+      updateRequired = false;
+      xSemaphoreTake(renderingMutex, portMAX_DELAY);
+      render();
+      xSemaphoreGive(renderingMutex);
+    }
+    vTaskDelay(10 / portTICK_PERIOD_MS);
+  }
+}
+
+void HomeActivity::render() const {
+  renderer.clearScreen();
+
+  const auto pageWidth = GfxRenderer::getScreenWidth();
+  const auto pageHeight = GfxRenderer::getScreenHeight();
+  renderer.drawCenteredText(READER_FONT_ID, 10, "CrossPoint Reader", true, BOLD);
+
+  // Draw selection
+  renderer.fillRect(0, 60 + selectorIndex * 30 + 2, pageWidth - 1, 30);
+  renderer.drawText(UI_FONT_ID, 20, 60, "Read", selectorIndex != 0);
+  renderer.drawText(UI_FONT_ID, 20, 90, "Settings", selectorIndex != 1);
+
+  renderer.drawRect(25, pageHeight - 40, 106, 40);
+  renderer.drawText(UI_FONT_ID, 25 + (105 - renderer.getTextWidth(UI_FONT_ID, "Back")) / 2, pageHeight - 35, "Back");
+
+  renderer.drawRect(130, pageHeight - 40, 106, 40);
+  renderer.drawText(UI_FONT_ID, 130 + (105 - renderer.getTextWidth(UI_FONT_ID, "Confirm")) / 2, pageHeight - 35,
+                    "Confirm");
+
+  renderer.drawRect(245, pageHeight - 40, 106, 40);
+  renderer.drawText(UI_FONT_ID, 245 + (105 - renderer.getTextWidth(UI_FONT_ID, "Left")) / 2, pageHeight - 35, "Left");
+
+  renderer.drawRect(350, pageHeight - 40, 106, 40);
+  renderer.drawText(UI_FONT_ID, 350 + (105 - renderer.getTextWidth(UI_FONT_ID, "Right")) / 2, pageHeight - 35, "Right");
+
+  renderer.displayBuffer();
+}