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/EpubReaderActivity.h

@@ -0,0 +1,35 @@
+#pragma once
+#include <Epub.h>
+#include <Epub/Section.h>
+#include <freertos/FreeRTOS.h>
+#include <freertos/semphr.h>
+#include <freertos/task.h>
+
+#include "../Activity.h"
+
+class EpubReaderActivity final : public Activity {
+  std::shared_ptr<Epub> epub;
+  std::unique_ptr<Section> section = nullptr;
+  TaskHandle_t displayTaskHandle = nullptr;
+  SemaphoreHandle_t renderingMutex = nullptr;
+  std::unique_ptr<Activity> subAcitivity = nullptr;
+  int currentSpineIndex = 0;
+  int nextPageNumber = 0;
+  int pagesUntilFullRefresh = 0;
+  bool updateRequired = false;
+  const std::function<void()> onGoBack;
+
+  static void taskTrampoline(void* param);
+  [[noreturn]] void displayTaskLoop();
+  void renderScreen();
+  void renderContents(std::unique_ptr<Page> p);
+  void renderStatusBar() const;
+
+ public:
+  explicit EpubReaderActivity(GfxRenderer& renderer, InputManager& inputManager, std::unique_ptr<Epub> epub,
+                              const std::function<void()>& onGoBack)
+      : Activity(renderer, inputManager), epub(std::move(epub)), onGoBack(onGoBack) {}
+  void onEnter() override;
+  void onExit() override;
+  void loop() override;
+};