Override onSetup when using a compatibility layer

The default `onSetup` will call `.begin`, to support initializing plugins using
the V1 plugins while using `KALEIDOSCOPE_INIT_PLUGINS`. However, plugins that
implement a compatibility layer so that they can be used with both the new API,
and with `Kaleidoscope.use()` will have a `.begin` method too. Which the default
`onSetup` will call, and we'll register the compatibility layer too, in addition
to the new-style event handlers. This results in many things running twice,
which leads to all kinds of problems.

For this reason, override `onSetup`, so that it does not call `begin`. When used
with `Kaleidoscope.use()`, the plugin will still work, so compatibility is
maintained. But the bug is now gone.

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

src/Kaleidoscope/Leader.h

@@ -50,6 +50,10 @@ class Leader : public kaleidoscope::Plugin {
   EventHandlerResult afterEachCycle();
 
 #if KALEIDOSCOPE_ENABLE_V1_PLUGIN_API
+  kaleidoscope::EventHandlerResult onSetup() {
+    return kaleidoscope::EventHandlerResult::OK;
+  }
+
  protected:
   void begin();
   static Key legacyEventHandler(Key mapped_key, byte row, byte col, uint8_t key_state);