Improve the compatibility layer

When the V1 plugin API is enabled, we need to override `onSetup`, to not call
`setup()` twice: once via the v2 default `onSetup`, and once via the legacy API.
Take special attention to call `LEDControl.mode_add`, so that the effect does
register as a LED mode.

Signed-off-by: Gergely Nagy <algernon@keyboard.io>

src/Kaleidoscope/AlphaSquare-Effect.h

@@ -29,6 +29,15 @@ class AlphaSquareEffect : public LEDMode {
   static uint16_t length;
 
   EventHandlerResult onKeyswitchEvent(Key &mappedKey, byte row, byte col, uint8_t keyState);
+
+#if KALEIDOSCOPE_ENABLE_V1_PLUGIN_API
+  kaleidoscope::EventHandlerResult onSetup() {
+    ::LEDControl.mode_add(this);
+
+    return kaleidoscope::EventHandlerResult::OK;
+  }
+#endif
+
  protected:
   void update(void) final;