kwin/convergentwindows: Don't maximize xwaylandvideobridge ghost window

Fix for #324 (closed)

The window is marked as "skipSwitcher", so we can hide it from the task switcher. The other problem is that the ghost window is getting maximized, we can't use the skipSwitcher property for that (since other windows we do want maximized that are marked with this). There isn't really any way to identify it so we will just add an exception for xwaylandvideobridge for now.