Ensure that Window::output() is always in sync with the frame geometry

plasmashell may see new output layout before kwin_x11 and move desktop
windows accordingly. If that happens, the desktop windows will have old
outputs in Window::output() property because the Workspace doesn't
re-check outputs if the output layout changes.

In order to fix the Window::output() property getting out of sync,
re-check the current output after the output layout changes. It should
also fix a visual glitch in the slide effect caused by
EffectWindow::screen() not being correct.