Fix theme application in various editor dialogs

99% of the time we shouldn't rely on the signal, we
should use the notification instead. I left some comments
in places where I couldn't quickly improve the code.

editor/export/project_export.h

@@ -188,7 +188,6 @@ class ProjectExportDialog : public ConfirmationDialog {
 	void _tab_changed(int);
 
 protected:
-	void _theme_changed();
 	void _notification(int p_what);
 	static void _bind_methods();