Do not allow to move GUI windows out of screen

MyGUI by default allows to move widgets out of screen borders, while Morrowind clamps coordinates of GUI widgets to make sure that they are present on the screen. Probably we should replicate this behaviour.

Also Morrowind forces you to keep a three-pixels gap between window border and GUI widget border, but I do not think that we should replicate it - it will cause issues with GUI scaling due to rounding errors.