embed nxproxy / support embedding of nxproxy in a control window

[sunweaver]

This would fix several issues reported against X2Go BTS:

Capturing special hotkeys and forwarding / not forwarding them to the remote session:
https://bugs.x2go.org/cgi-bin/bugreport.cgi?bug=838

Disable local screensaver if remote session goes into fullscreen mode (and enable that immediately once leaving it and the idle time has passed):
https://bugs.x2go.org/cgi-bin/bugreport.cgi?bug=554

It also will allow us to inject client-side windows on top of the nxproxy session window.