Disable multi-window buttons instead of hiding them when support is unavailable

This is more explicit as for why this functionality isn't available
depending on editor settings and current platform.

This also exposes a `EditorInterface.is_multi_window_enabled()` method
so that editor plugins can easily query whether the editor is able and
expected to create multiple windows.

editor/editor_interface.h

@@ -99,6 +99,7 @@ public:
 	void set_main_screen_editor(const String &p_name);
 	void set_distraction_free_mode(bool p_enter);
 	bool is_distraction_free_mode_enabled() const;
+	bool is_multi_window_enabled() const;
 
 	float get_editor_scale() const;