grim/gplugin

Add GPluginPluginInfo:provides and GPluginPluginInfo:priority properties

This is just the properties and none of the behavior yet. I'm doing this in two phases to make it easier to reason about as `gplugin_manager_refresh` is going to need some refactoring to make this all work.

Testing Done:
Ran the unit tests.

Bugs closed: GPLUGIN-79

Reviewed at https://reviews.imfreedom.org/r/691/
syntax: glob
.*.swp
*.debhelper
*.orig
*.pyc
*.ui~
\#*.ui\#
syntax: regexp
^build.*\/
^packaging\/debian\/tmp\/
^packaging\/debian\/(.+\.)?(substvars|debhelper\.log)$
^packaging\/debian\/build.*\/
^packaging\/debian\/libgplugin0?(-.+)?\/
^packaging\/debian\/gir1.2-gplugin-0.0\/
^packaging\/debian\/files$
^po\/.*\.pot$
^obj-x86_64-linux-gnu\/
^debian\/?$