Activate clients on drag enter
We currently only raise a client when a drag enters it and not activate it.
This is confusing since afterwards the raised window has not keyboard focus
although visually being in front of the window the drag originated from.
Therefore activate entered windows instead of only raising them.
Test Plan: Manually and autotests still pass.
Reviewers: KWin, hein
Reviewed By: hein
Subscribers: anthonyfieroni, kwin
Differential Revision: https://phabricator.kde.org/D15225