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