Add a dialpad UI for PSTN lookup

Queries the homeserver for PSTN protocol support, and if found,
the add-room button on the DM rooms list section opens a context
menu instead with a 'dial pad' option as well as the current 'start chat'
dialog. Entering a number into this and pressing dial performs
a thirdparty user query for the given string and starts a DM with that
user.

src/dispatcher/actions.ts

@@ -94,4 +94,15 @@ export enum Action {
      * Trigged after the phase of the right panel is set. Should be used with AfterRightPanelPhaseChangePayload.
      */
     AfterRightPanelPhaseChange = "after_right_panel_phase_change",
+
+    /**
+     * Opens the modal dial pad
+     */
+    OpenDialPad = "open_dial_pad",
+
+    /**
+     * Fired when CallHandler has checked for PSTN protocol support
+     * XXX: Is an action the right thing for this?
+     */
+    PstnSupportUpdated = "pstn_support_updated",
 }