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/shader_create_dialog.h

@@ -101,7 +101,6 @@ class ShaderCreateDialog : public ConfirmationDialog {
 	void _update_dialog();
 
 protected:
-	void _update_theme();
 	void _notification(int p_what);
 	static void _bind_methods();