Add a focus_composer dispatcher action

and use it

src/dispatcher/actions.ts

@@ -53,4 +53,9 @@ export enum Action {
      * Provide status information for an ongoing update check. Should be used with a CheckUpdatesPayload.
      */
     CheckUpdates = "check_updates",
+
+    /**
+     * Focuses the user's cursor to the composer. No additional payload information required.
+     */
+    FocusComposer = "focus_composer",
 }