Delayed focus updates for Toplevels without surface

Summary:
XWayland clients have a surface associated with them asynchronously. In this
case we don't directly want to set focus on this Toplevel, but wait until the
surface is set.

This patch aims in conjunction with an unrelated fix to SDL at improving Steam
Big Picture Mode in our Wayland session.

Test Plan: Steam BPM regains focus on game close.

Reviewers: KWin, zzag

Reviewed By: KWin, zzag

Subscribers: anthonyfieroni, davidedmundson, zzag, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D19262