Reorganization of library structure to better match Arduino spec (#20)

Note, this will also require to you "uninstall and reinstall" the Arduino library due to the library file location changes.

Reviewed-on: https://git.pinkduck.xyz/awonak/libGravity/pulls/20

firmware/Gravity/save_state.h

@@ -13,7 +13,7 @@
 #define SAVE_STATE_H
 
 #include <Arduino.h>
-#include <gravity.h>
+#include <libGravity.h>
 
 // Forward-declare AppState to avoid circular dependencies.
 struct AppState;