Add a DBus Interface to change the window/output count in nested mode

At the moment it is possible to pass the number of windows as command line argument, removing outputs can be simulated by closing a window from the outside, but there is no way to open a new one. This adds a new dbus interface when kwin_wayland is running inside X or another compositor that makes it possible to change the number of windows programatically and can so simulate output removals helping testing multi screen scenarios. Also adds a method to precisely chage the size of a window instead of reliying on manual window resizes from the outside. This can also be useful when running in a session where there is no way to adjust the window size from the outside, as it is impossible to change it from inside the nested session currently.