Add option to rotate the display (#27)

I needed to cut the bootsplash to make room for adding this features.

Reviewed-on: https://git.pinkduck.xyz/awonak/libGravity/pulls/27

firmware/Gravity/app_state.h

@@ -31,6 +31,7 @@ struct AppState {
     Clock::Pulse selected_pulse = Clock::PULSE_PPQN_24;
     bool editing_param = false;
     bool encoder_reversed = false;
+    bool rotate_display = false;
     bool refresh_screen = true;
 };