add support for spectrum plugin to custom status bar

[fossfreedom]

if installed, spectrum can be displayed on the right-hand side of the custom status bar (its just a Gtk.Box I think).

Doubt anything can be done with the standard status bar - possibly using the alternative toolbar search trick - but lets separate this into a separate issue

---

Want to back this issue? Post a bounty on it! We accept bounties via Bountysource.