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/