Editor: update properties referencing GUIs on GUI removal or id change

[ivan-mogilko]

Fix #2542

1. Properties that let select GUI display a dropdown list of gui script names instead of being raw number fields. The displayed names are mapped to GUI number internally, so no data upgrade is necessary.
2. Whenever a GUI is removed, or has ID changed by command, SettingsComponent class updates all of its properties and fixes GUI selection. If selected GUI got removed, property is changed to 0 ("None").

The feature is purely in UI and does not change data format.

[ivan-mogilko]

I used existing Game.GUIAddedOrRemoved event, and had to add a special event for gui ID changing to GuiComponent.
But I think that these events (both old and new) are a hack. IMO we need some sort of a IGameListener (made up name) interface that would contain all similar events, and through which various managers would notify others about changes they made. I do not have spare time to design something like that right now. Added comments with this idea into the code.