Add is_exiting to the EditorInterface

[Naros]

There are some corner cases where an EditorPlugin may need to know whether the editor is currently in an exit sequence, and to perform some different behavior in this case. There is currently no reliable way to infer that state without resorting to scene node hacks to check for various things such as "is editor base control dimmed".

[ydeltastar]

What is the difference from using _exit_tree() in the EditorPlugin?

[Naros]

@ydeltastar here's an example of what a real GDExtension plugin may implement, using ScriptEditor as an example:

void ScriptEditorPlugin::save_external_data() {
	if (!EditorNode::get_singleton()->is_exiting()) {
		script_editor->save_all_scripts();
	}
}

Right now this requires silly hacks like this:

    const bool editor_exiting = EditorInterface::get_singleton()->get_base_control()->get_modulate() != Color(1, 1, 1, 1);
    if (!editor_exiting && _editor_panel)
        _editor_panel->save_all_resources();

If how Editor dimming changes in the future, that logic breaks and becomes brittle. A reliable and consistent way should be provided so that the same sort of behavior can be achieved in GDExtension.

I think relying on _exit_tree may work in some circumstances, but it would not here because the editor calls back into the _save_external_data before the EditorPlugin is removed from the scene tree.