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_egui/src/lib.rs

@@ -89,7 +89,7 @@ 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>> {
         let params = self.params.clone();
         let peak_meter = self.peak_meter.clone();
         create_egui_editor(