Fix space switching on mobile

We forgot to connect to onTapped on the touchscreen-specific TapHandler.
I also needed to carve out a specific case for DMs so it doesn't try to
resolve the non-existent "DM" resource.

src/app/qml/AvatarTabButton.qml

@@ -48,6 +48,7 @@ Delegates.RoundedItemDelegate {
 
     TapHandler {
         acceptedDevices: PointerDevice.TouchScreen
+        onTapped: root.selected()
         onLongPressed: root.contextMenuRequested()
     }