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/ActivityWithSubactivity.h

@@ -0,0 +1,17 @@
+#pragma once
+#include <memory>
+
+#include "Activity.h"
+
+class ActivityWithSubactivity : public Activity {
+ protected:
+  std::unique_ptr<Activity> subActivity = nullptr;
+  void exitActivity();
+  void enterNewActivity(Activity* activity);
+
+ public:
+  explicit ActivityWithSubactivity(GfxRenderer& renderer, InputManager& inputManager)
+      : Activity(renderer, inputManager) {}
+  void loop() override;
+  void onExit() override;
+};