Manage plugins and motifs from Add-ons Manager

[Ascrod]

It would be nice to be able to manage plugins and motifs from the add-ons manager. This would entail integrating with the existing add-ons page, similar to how Stylem and GreaseMonkey do. The Light and Dark motifs would be installed by default, and only one motif would be enabled at a time. Additionally, motifs and plugins will need to include a header block to supply meta data - again, similar to what Stylem and GreaseMonkey require. The plugins tab can use the existing "Install Plugin" UI, as well.

This would not be a trivial change, and will likely break existing plugins and motifs, so care would need to be taken.