[kde-workspace] /: block until the lock windows are shown; prevents sleeping before locking

[Oliver Henshaw]

> So this might explain why the screen is sometimes only fully locked
> after resuming from suspend:
> 1. The greeterapp blocks first on ksmserver so doesn't enter
> desktopResized until Interface::Lock() returns to the event loop ...
> 2. ...and then blocks on powerdevil inside desktopResized(). By this
> time powerdevil may have triggered the suspend.

See also https://bugs.kde.org/show_bug.cgi?id=313123#c13 onwards.