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/ReaderActivity.h

@@ -0,0 +1,24 @@
+#pragma once
+#include <memory>
+
+#include "../ActivityWithSubactivity.h"
+
+class Epub;
+
+class ReaderActivity final : public ActivityWithSubactivity {
+  std::string initialEpubPath;
+  const std::function<void()> onGoBack;
+  static std::unique_ptr<Epub> loadEpub(const std::string& path);
+
+  void onSelectEpubFile(const std::string& path);
+  void onGoToFileSelection();
+  void onGoToEpubReader(std::unique_ptr<Epub> epub);
+
+ public:
+  explicit ReaderActivity(GfxRenderer& renderer, InputManager& inputManager, std::string initialEpubPath,
+                          const std::function<void()>& onGoBack)
+      : ActivityWithSubactivity(renderer, inputManager),
+        initialEpubPath(std::move(initialEpubPath)),
+        onGoBack(onGoBack) {}
+  void onEnter() override;
+};