This is an initial, static implementation of plugins. The
CallsApplication has a plugin name which can be changed with a new
--provider command line option. This plugin name is used to
instantiate the appropriate plugin when the application is activated.
From then on, the plugin cannot change.
In future, we can expand this support to include loading multiple
plugins at once, configurable through some UI. This will have
far-reaching implications though, and complicate things like
enumerating the provider hierarchy. There is also no practical
benefit right now; the mm and ofono plugins can't be used at the same
time because ModemManager and oFono don't play nice together, and the
whole raison d'être of the dummy plugin is undermined if you can make
use of one of the others. So for now, we just implement one static
plugin.
