Windows: Don't cover the taskbar when maximizing frameless windows

Brings Windows QPA on par with other platforms.

[ChangeLog][Windows] Don't cover the taskbar when maximizing
frameless windows.

Task-number: QTBUG-8361
Change-Id: Iba35132f697cb7379650a4c883b616c5c2023d4c
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>