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>

gravity.h

@@ -14,6 +14,8 @@
 // Hardware abstraction wrapper for the Gravity module.
 class Gravity {
    public:
+    static const uint8_t OUTPUT_COUNT = 6;
+
     // Constructor
     Gravity()
         : display(U8G2_R2, SCL, SDA, U8X8_PIN_NONE) {}
@@ -27,7 +29,7 @@ class Gravity {
     // Polling check for state change of inputs and outputs.
     void Process();
 
-    U8G2_SSD1306_128X64_NONAME_2_HW_I2C display;  // OLED display object.
+    U8G2_SSD1306_128X64_NONAME_1_HW_I2C display;  // OLED display object.
     Clock clock;                                  // Clock source wrapper.
     DigitalOutput outputs[OUTPUT_COUNT];          // An array containing each Output object.
     EncoderDir encoder;                           // Rotary encoder with button instance