Use new GLib application API for launching

Use new GLib application API for launching

Launch child processes more directly; we retrieve the PID, and
use it to keep track of the .desktop file we launched.

Now, when we get a window, since the X window has a PID, we
have a pretty strong association.

.desktop file <-> PID <-> window

And can thus map window back to .desktop file.

https://bugzilla.gnome.org/show_bug.cgi?id=637745