Add settings button to popover

[bonjorno7]

Right now the settings button is next to Autosave Format in the popover, but only when using Autosave Custom.
I think it's useful to be able to quickly access the settings for an addon from its UI, but don't have a good place to put it in the respective main panels.
An acceptable solution would be to just put it in the header of the popover, and have the settings button open the preferences for whichever addon was currently selected.
Though I'm not sure what should happen if you select an addon you don't have enabled.