Lots of changes and optimizations

- add reverse encoder menu option and save state
- improve usage of EncoderDir in ISR with pointer to instance and static isr() method.
- reduce u8g2 memory usage by using single page buffer
- improve save state behavor by using a mutex flag and update check with debounce in main loop
- make saving to EEPROM safer by wrapping put calls with noInterrupts()

examples/Gravity/app_state.h

@@ -8,6 +8,7 @@
 // Global state for settings and app behavior.
 struct AppState {
     int tempo = Clock::DEFAULT_TEMPO;
+    bool encoder_reversed = false;
     bool refresh_screen = true;
     bool editing_param = false;
     int selected_param = 0;