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

src/shell-window-tracker-private.h

@@ -6,4 +6,8 @@
 
 void _shell_window_tracker_notify_app_state_changed (ShellWindowTracker *tracker, ShellApp *self);
 
+void _shell_window_tracker_add_child_process_app (ShellWindowTracker *tracker,
+                                                  GPid                pid,
+                                                  ShellApp           *app);
+
 #endif