core: Allow fullscreen modals to move out of the center of their parents
If a modal dialog (i.e. with a "revolves around center of parent window" policy) becomes fullscreen, we cannot neatly honor both the modal dialog going fullscreen and the window staying around the center of its parent.
In order to make fullscreening work in this situation, allow fullscreen modal dialogs to "snap out" of the parent. This rule will become again effective after the window is unfullscreened.
Closes: #3425 (closed)