DolphinWX: Add a progress dialog host command

Allows feedback from backends to be communicated to the user when
long-running operation are performed (e.g. shader compilation).

Source/Core/DolphinWX/Globals.h

@@ -307,6 +307,7 @@ enum
   IDM_WINDOW_SIZE_REQUEST,
   IDM_STOPPED,
   IDM_HOST_MESSAGE,
+  IDM_UPDATE_PROGRESS_DIALOG,
 
   IDM_MPANEL,
   ID_STATUSBAR,