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/Activity.h

@@ -0,0 +1,18 @@
+#pragma once
+#include <InputManager.h>
+
+class GfxRenderer;
+
+class Activity {
+ protected:
+  GfxRenderer& renderer;
+  InputManager& inputManager;
+
+ public:
+  explicit Activity(GfxRenderer& renderer, InputManager& inputManager)
+      : renderer(renderer), inputManager(inputManager) {}
+  virtual ~Activity() = default;
+  virtual void onEnter() {}
+  virtual void onExit() {}
+  virtual void loop() {}
+};