Fix evaluating window rules for closed windows

gedit changes its caption from "Untitled document - gedit" to "gedit" when it closes. This schedules Window::evaluateWindowRules() to be called when the window is already marked as deleted. kwin then crashes.

In order to prevent that, a direct connection can be used instead. But then the caption must be initialized extra carefully because if the window rule changes the window type, "" can be lost.