Added alternate method to update live_composite_keymap_

This adds a new version of `updateLiveCompositeKeymap()` which takes three parameters:
row, column, and a `Key` value. Instead of looking up the value to update in the key map,
it just updates `live_composite_keymap_` with the specified `Key` value.

Signed-off-by: Michael Richters <gedankenexperimenter@gmail.com>

src/kaleidoscope/layers.h

@@ -99,6 +99,9 @@ class Layer_ {
   static Key getKeyFromPROGMEM(uint8_t layer, byte row, byte col);
 
   static void updateLiveCompositeKeymap(byte row, byte col);
+  static void updateLiveCompositeKeymap(byte row, byte col, Key mappedKey) {
+    live_composite_keymap_[row][col] = mappedKey;
+  }
   static void updateActiveLayers(void);
 
  private: