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/CrossPointState.h

@@ -3,11 +3,20 @@
 #include <string>
 
 class CrossPointState {
+  // Static instance
+  static CrossPointState instance;
+
  public:
   std::string openEpubPath;
   ~CrossPointState() = default;
 
+  // Get singleton instance
+  static CrossPointState& getInstance() { return instance; }
+
   bool saveToFile() const;
 
   bool loadFromFile();
 };
+
+// Helper macro to access settings
+#define APP_STATE CrossPointState::getInstance()