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/CrossPointState.cpp

@@ -6,8 +6,12 @@
 
 #include <fstream>
 
+namespace {
 constexpr uint8_t STATE_FILE_VERSION = 1;
 constexpr char STATE_FILE[] = "/sd/.crosspoint/state.bin";
+}  // namespace
+
+CrossPointState CrossPointState::instance;
 
 bool CrossPointState::saveToFile() const {
   std::ofstream outputFile(STATE_FILE);