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/UnitTests/StubHost.cpp

@@ -57,6 +57,9 @@ void Host_ShowVideoConfig(void*, const std::string&)
 void Host_YieldToUI()
 {
 }
+void Host_UpdateProgressDialog(const char* caption, int position, int total)
+{
+}
 std::unique_ptr<cInterfaceBase> HostGL_CreateGLInterface()
 {
   return nullptr;