fix: add missing Qt/Gui includes for ThemeSelectorWidget

Add #include <QApplication> (for qApp macro), #include <QEvent> (for
QEvent::LanguageChange), and #include <Gui/Application.h> (for
Gui::Application::Instance) which were transitively available in
FreeCAD 1.0 but not in 1.2.

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;