Fix DesktopGrid drag on X11
Summary:
EffectsAPI explicitly says:
"On X11, the window will end up on the last window in the list" and
DesktopGrid reliaed on that.
Using the last makes sense as it means the
enterDesktop method will work for both.
Somehow in the refactors AbstractClient ended up doing the opposite.
Reviewers: KWin, zzag
Reviewed By: KWin, zzag
Subscribers: zzag, kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D18339