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.cpp

@@ -0,0 +1,21 @@
+#include "ActivityWithSubactivity.h"
+
+void ActivityWithSubactivity::exitActivity() {
+  if (subActivity) {
+    subActivity->onExit();
+    subActivity.reset();
+  }
+}
+
+void ActivityWithSubactivity::enterNewActivity(Activity* activity) {
+  subActivity.reset(activity);
+  subActivity->onEnter();
+}
+
+void ActivityWithSubactivity::loop() {
+  if (subActivity) {
+    subActivity->loop();
+  }
+}
+
+void ActivityWithSubactivity::onExit() { exitActivity(); }