Add support for _GTK_FRAME_EXTENTS

Summary:
_GTK_FRAME_EXTENTS is a property set by the client that specifies extents
of the client-side drop-shadow, which should be ignored when doing things
like resizing, placing, snapping to screen borders, etc.

_GTK_FRAME_EXTENTS is a proprietary GTK atom, it's not in the NetWM spec.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D24657