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/Core/Host.h

@@ -35,6 +35,7 @@ void Host_UpdateMainFrame();
 void Host_UpdateTitle(const std::string& title);
 void Host_ShowVideoConfig(void* parent, const std::string& backend_name);
 void Host_YieldToUI();
+void Host_UpdateProgressDialog(const char* caption, int position, int total);
 
 // TODO (neobrain): Remove this from host!
 void* Host_GetRenderHandle();