Add filtering and exploring to the new room list

This is per the designs. Animation doesn't feel required here.

Like the rest of this series, this rewrites a component to be more purpose-built to help match the designs and to solve the smallest possible problem.

src/dispatcher/actions.ts

@@ -40,6 +40,11 @@ export enum Action {
      */
     ViewUserSettings = "view_user_settings",
 
+    /**
+     * Opens the room directory. No additional payload information required.
+     */
+    ViewRoomDirectory = "view_room_directory",
+
     /**
      * Sets the current tooltip. Should be use with ViewTooltipPayload.
      */