Fix placement of windows on disconnected outputs

If an output is disconnected, the Workspace will update the
Toplevel.output property for all windows that are on that output, then
it will call AbstractClient::checkWorkspacePosition() to fix window
position.

That may result in some windows partially sticking outside visible area
because AbstractClient::checkWorkspacePosition() has no idea what output
the window was.

This change addresses that problem by delaying updating the
Toplevel.output property so AbstractClient::checkWorkspacePosition()
could pick better window placement.