Investigate removing the custom window system and swapping to Godot 4 default window nodes

[hhyyrylainen]

as those are hopefully fixed enough to no longer require any of the workarounds we needed in Godot 3 and made our own window system.

including the CustomDialog class.

[hhyyrylainen]

I'm marking this as high priority because if this is done then all game GUI will need to be re-checked and updated so this is better to do earlier than later when the GUI is already fixed.