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.cpp

@@ -11,6 +11,10 @@
 
 #include "gravity.h"
 
+// Initialize the static pointer for the EncoderDir class to null. We want to
+// have a static pointer to decouple the ISR from the global gravity object. 
+EncoderDir* EncoderDir::_instance = nullptr;
+
 void Gravity::Init() {
     initClock();
     initInputs();
@@ -68,18 +72,13 @@ void Gravity::Process() {
     }
 }
 
-void ReadEncoder() {
-    gravity.encoder.UpdateEncoder();
-}
-
-// Define Encoder pin ISR.
-// Pin Change Interrupt on Port C (D17/A3).
-ISR(PCINT2_vect) {
-    ReadEncoder();
-};
 // Pin Change Interrupt on Port D (D4).
+ISR(PCINT2_vect) {
+    EncoderDir::isr();
+};
+// Pin Change Interrupt on Port C (D17/A3).
 ISR(PCINT1_vect) {
-    ReadEncoder();
+    EncoderDir::isr();
 };
 
 // Singleton