Configurable keybindings for x2go(agent)

Hello,

NX annoyingly adds hardcoded keybindings which overlap with e.g.
GNOME's. Where in GNOME, Ctrl-Alt-t opens a new terminal, in NX and
therefore in x2go, it closes the session.

Therefore I hacked together configurable keybinding support for
nxagent/x2goagent as described here:
http://wiki.x2go.org/doku.php/wiki:components:nxagent_keyboard_shortcuts

Currently this is experimental, so please test and report bugs, if there
are no major breakages, you can expect it for version 3.5.0.17.

Ciao,

Alexander Wuerstlein.