Introduce StateManager to persist state between power cycles (#6)

- add reset state menu option to return all settings back to default values.
- add reverse encoder menu option and save state
- make saving to EEPROM safer by wrapping put calls with noInterrupts()
- improve save state behavior by using a mutex flag and update check with debounce in main loop
- refactor gravity.h global const definitions to be static and more readable.
- improve usage of EncoderDir in ISR with pointer to instance and static isr() method.
- reduce u8g2 memory usage by using single page buffer

Reviewed-on: https://git.pinkduck.xyz/adam/libGravity/pulls/6
Co-authored-by: Adam Wonak <adam.wonak@gmail.com>
Co-committed-by: Adam Wonak <adam.wonak@gmail.com>

examples/Gravity/app_state.h

@@ -0,0 +1,21 @@
+#ifndef APP_STATE_H
+#define APP_STATE_H
+
+#include <gravity.h>
+
+#include "channel.h"
+
+// 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;
+    int selected_sub_param = 0;
+    byte selected_channel = 0;  // 0=tempo, 1-6=output channel
+    Clock::Source selected_source = Clock::SOURCE_INTERNAL;
+    Channel channel[Gravity::OUTPUT_COUNT];
+};
+
+#endif  // APP_STATE_H