Added function to return the active layer of a given key

New function `lookupActiveLayer()` returns the number of the layer
that a given key (`row`, `col`) is currently mapped from.

src/layers.h

@@ -52,6 +52,9 @@ class Layer_ {
     uint8_t layer = activeLayers[row][col];
     return (*getKey)(layer, row, col);
   }
+  static uint8_t lookupActiveLayer(byte row, byte col) {
+    return activeLayers[row][col];
+  }
   static void on(uint8_t layer);
   static void off(uint8_t layer);
   static void move(uint8_t layer);