Don't overwrite window title when attaching to xpra display

[cryzed]

Currently when firejail attaches to the created xpra display it overwrites the window title with "firejail x11 sandbox". I realize that it's important for users to know if the window is X11 sandboxed or not, but using a xpra string-template that preserves the window title would be much nicer (e.g. @title@ (Firejail X11 Sandbox)), or alternatively, not messing with the window title at all and let users customize this option either in /etc/xpra/xpra.conf or using xpra-extra-params in /etc/firejail/firejail.config. At the moment any custom setting is simply overriden, resulting in basically useless window titles.

[netblue30]

I'll put something in to fix it. We already have this feature for Xephyr in /etc/firejail/firejail.config

[rusty-snake]

Backlink #3331