gdesktopappinfo: Add g_desktop_app_info_launch_uris_as_manager()

A new GDesktopAppInfo specific function which provides more control
over launched processes.  Intended basically only for use in GNOME
Shell, where we want:

*) To directly know the GPid for each launched program, without
   having to listen to a DBus signal emitted in our own process
*) Possibly control over the process environment; for example,
   we may want to call setsid() or redirect file descriptors.

And in the future:
*) To avoid recursively calling ourself via DBus, when a later
   patch causes g_app_info_launch() to indirect via the shell.

https://bugzilla.gnome.org/show_bug.cgi?id=606960