Add per-channel Swing configuration (#7)

Select swing amount from a percentage range of the beat starting a 50% (unchanged) to a max swing amount of 95% (about 1/32nd note before end of period). Swing percentage shows an indicator marker when the percentage lines up with a quantized note on the grid.

This is probably going to be the last feature because it is pushing up against the limits of available dynamic memory.

Out of scope changes:
- selecting parameters / values no longer wraps
- reduce dynamic memory used in processClockTick
- various readability formatting

Reviewed-on: https://git.pinkduck.xyz/adam/libGravity/pulls/7
Co-authored-by: Adam Wonak <adam.wonak@gmail.com>
Co-committed-by: Adam Wonak <adam.wonak@gmail.com>

examples/Gravity/app_state.h

@@ -14,6 +14,7 @@ struct AppState {
     int selected_param = 0;
     int selected_sub_param = 0;
     byte selected_channel = 0;  // 0=tempo, 1-6=output channel
+    byte selected_shuffle = 0;
     Clock::Source selected_source = Clock::SOURCE_INTERNAL;
     Channel channel[Gravity::OUTPUT_COUNT];
 };
@@ -37,6 +38,7 @@ enum ParamsChannelPage {
     PARAM_CH_PROB,
     PARAM_CH_DUTY,
     PARAM_CH_OFFSET,
+    PARAM_CH_SWING,
     PARAM_CH_CV_SRC,
     PARAM_CH_CV_DEST,
     PARAM_CH_LAST,