Factored out ipcRenderer.on

Summary:
Delegate receiving events from the Electron host process to the RenderHost interface.

Typed them as well while at it.

Reviewed By: timur-valiev

Differential Revision: D31828130

fbshipit-source-id: 920055de6f48512b91a433b229dbacd611c6f139

desktop/app/src/init.tsx

@@ -297,5 +297,10 @@ function initializeFlipperForElectron() {
     hasFocus() {
       return remote.getCurrentWindow().isFocused();
     },
+    onIpcEvent(event, callback) {
+      ipcRenderer.on(event, (_ev, ...args: any[]) => {
+        callback(...(args as any));
+      });
+    },
   });
 }