Move the existing refresh code to GPluginFileSource

I was hoping this would required a bit less tweaking, but it's functionally the
same as the unit tests pass, but things got weird because we're using the
GObject life cycle to mimic stuff we just had scoped differently in the preivous
version.

Testing Done:
Ran the unittests, pulled some hair out, complained on stream...

Reviewed at https://reviews.imfreedom.org/r/1788/