Default sessions from /usr/share/xsessions instead of /etc/X11/Sessions

[lahwaacz]

I think it would be better to support loading of default sessions from /usr/share/xsessions instead the current /etc/X11/Sessions, which is not standardized (I think) and presumes that the user creates the scripts. Many window managers install their .desktop file under /usr/share/xsessions, so it's definitely better.

[fyra]

I started work on this, but I'm uncertain as to what is likely to be in these .desktop-files and whether looking for the "Exec=" part will be enough.