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/
The following individuals have contributed to this project.
Ankit Vani <a@nevitus.org>
EionRobb <eion@robbmob.com>
Elliott Sales de Andrade <quantum.analyst@gmail.com>
Gary Kramlich <grim@reaperworld.com>
Jason Scherer <schereja@gmail.com>
John Bailey <rekkanoryo@rekkanoryo.org>
Mike Ruprecht <cmaiku@gmail.com>
Olaf Hering <olaf@aepfle.de>
Richard Laager <rlaager@pidgin.im>
Sorvival <sorvival@tutanota.com>