st: Use g_object_notify_by_pspec()

st: Use g_object_notify_by_pspec()

`g_object_notify()` actually takes a global lock to look up the property
by its name, which means there is a performance hit (albeit tiny) every
time this function is called. For this reason, always try to use
`g_object_notify_by_pspec()` instead.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/652