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>

clock.h

@@ -23,22 +23,22 @@
 #define MIDI_STOP 0xFC
 #define MIDI_CONTINUE 0xFB
 
-const int DEFAULT_TEMPO = 120;
-
 typedef void (*ExtCallback)(void);
 static ExtCallback extUserCallback = nullptr;
 static void serialEventNoop(uint8_t msg, uint8_t status) {}
 
-enum Source {
-    SOURCE_INTERNAL,
-    SOURCE_EXTERNAL_PPQN_24,
-    SOURCE_EXTERNAL_PPQN_4,
-    SOURCE_EXTERNAL_MIDI,
-    SOURCE_LAST,
-};
-
 class Clock {
    public:
+    static constexpr int DEFAULT_TEMPO = 120;
+
+    enum Source {
+        SOURCE_INTERNAL,
+        SOURCE_EXTERNAL_PPQN_24,
+        SOURCE_EXTERNAL_PPQN_4,
+        SOURCE_EXTERNAL_MIDI,
+        SOURCE_LAST,
+    };
+
     void Init() {
         NeoSerial.begin(31250);