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/util/FullScreenMessageActivity.h

@@ -0,0 +1,21 @@
+#pragma once
+#include <EInkDisplay.h>
+#include <EpdFontFamily.h>
+
+#include <string>
+#include <utility>
+
+#include "../Activity.h"
+
+class FullScreenMessageActivity final : public Activity {
+  std::string text;
+  EpdFontStyle style;
+  EInkDisplay::RefreshMode refreshMode;
+
+ public:
+  explicit FullScreenMessageActivity(GfxRenderer& renderer, InputManager& inputManager, std::string text,
+                                     const EpdFontStyle style = REGULAR,
+                                     const EInkDisplay::RefreshMode refreshMode = EInkDisplay::FAST_REFRESH)
+      : Activity(renderer, inputManager), text(std::move(text)), style(style), refreshMode(refreshMode) {}
+  void onEnter() override;
+};