Add support for /usr/share/xsessions.

This adds detection and simple parsing support for `.desktop` files in `/usr/share/xsessions`.

This patch implements it as last place to look, when $binlist is not set. It might be a good idea to change this in the future to either merge the list with `/etc/X11/Sessions` sessions, or `.desktop` files having precedence over them.

Tested on Ubuntu 12.10 with a small sample of window managers.