Add an AsyncExecutor for editor GUIs

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

plugins/spectral_compressor/src/lib.rs

@@ -278,7 +278,7 @@ impl Plugin for SpectralCompressor {
         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.editor_state.clone())
     }