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/CrossPointSettings.cpp

@@ -10,9 +10,11 @@
 // Initialize the static instance
 CrossPointSettings CrossPointSettings::instance;
 
+namespace {
 constexpr uint8_t SETTINGS_FILE_VERSION = 1;
 constexpr uint8_t SETTINGS_COUNT = 2;
 constexpr char SETTINGS_FILE[] = "/sd/.crosspoint/settings.bin";
+}  // namespace
 
 bool CrossPointSettings::saveToFile() const {
   // Make sure the directory exists