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.cpp

@@ -0,0 +1,14 @@
+#include "FullScreenMessageActivity.h"
+
+#include <GfxRenderer.h>
+
+#include "config.h"
+
+void FullScreenMessageActivity::onEnter() {
+  const auto height = renderer.getLineHeight(UI_FONT_ID);
+  const auto top = (GfxRenderer::getScreenHeight() - height) / 2;
+
+  renderer.clearScreen();
+  renderer.drawCenteredText(UI_FONT_ID, top, text.c_str(), true, style);
+  renderer.displayBuffer(refreshMode);
+}