Add an AsyncExecutor for editor GUIs

This is decoupled form `GuiContext` as that would require invasive
changes all over the place.

plugins/examples/gain_gui_vizia/src/lib.rs

@@ -85,7 +85,7 @@ impl Plugin for Gain {
         self.params.clone()
     }
 
-    fn editor(&self) -> Option<Box<dyn Editor>> {
+    fn editor(&self, _async_executor: AsyncExecutor<Self>) -> Option<Box<dyn Editor>> {
         editor::create(
             self.params.clone(),
             self.peak_meter.clone(),