Fix editor layout reset on startup

[Hilderin]

• Fixes Project opens empty sometimes (no open scene and layout is reset) #96561

I'm pretty sure it's a regression from #93064. While loading the docks, EditorDockManager::_update_layout calls EditorNode::get_singleton()->save_editor_layout_delayed() before the editor finishes loading the last editor layout. This starts a timer to save the layout after 0.5 seconds. With the addition of a progress dialog in EditorNode::_load_editor_layout, the timer started in save_editor_layout_delayed can now be executed before the editor finishes loading, as the signal can be processed during Main::Iteration.

I added a new global boolean load_editor_layout_done to prevent the editor layout from being saved before it is fully loaded.

[akien-mga]

Thanks!