Fix ThemeSelectorWidget build: add missing includes

Add QApplication (for qApp macro), QEvent (for QEvent::LanguageChange),
and Gui/Application.h (for Gui::Application::Instance) which were
missing after the develop merge.

src/Mod/Start/Gui/ThemeSelectorWidget.cpp

@@ -21,6 +21,8 @@
  *                                                                          *
  ***************************************************************************/
 
+#include <QApplication>
+#include <QEvent>
 #include <QHBoxLayout>
 #include <QLabel>
 #include <QString>
@@ -28,6 +30,7 @@
 
 #include "ThemeSelectorWidget.h"
 #include <gsl/pointers>
+#include <Gui/Application.h>
 #include <Gui/PreferencePackManager.h>
 
 using namespace StartGui;