This is kind of in a middle ground at the moment. Even though it
handles sequences not coming from libsn, they're added nowhere at
the moment, we'll rely on the app launch context being in the x11
side at the moment.

Also, even though we do create internal sequence objects, we keep
exposing SnStartupSequences to make gnome-shell happy, we could
consider making this object "public" (and the sequence objects with
it), things stay private at the moment.

https://bugzilla.gnome.org/show_bug.cgi?id=762268