This includes the following changes:
- Introduce a `providers` hash table to keep track of multiple CallProvider's
and remove the `provider` member
- Remove `calls_manager_get_provider()` and `calls_manager_set_provider()`
in favour of `calls_manager_add_provider()`, `calls_manager_remove_provider()`
and `calls_manager_has_provider()`
- Introduce a `origins` GListStore to keep track of available origins.
`origins` is updated in `items_changed_cb()` when the origins of any
CallsProvider are updated.
- Adapt to changes with respect to `calls_manager_get_origins()`.
- Introduce `origins_by_protocol` hash table to keep track of available origins
per protocol.
- Adjust tests
- We temporarily break country code lookup which was handled previously
with the "default-origin" mechanism. We will add it back to the CallsSettings
class which will provide a better application-wide mechanism.
This is a bit of a hack to get the country code property hooked up
and will require a rework when multiple provider support is being added.
As the default origin is not being used anywhere else in the code
it's currently not a problem that we're not tracking if the default
origin is being removed.
Additionally test-manager.c needed some adjustment.
And adapt to changes
It's easier to track changes if we are using GListModel than
a GList. Also, eventually we shall have multiple providers,
and we can use GtkFlattenListModel to merge multiple origins
without manually managing pointers ourself.
Since several widgets has APIs to handle GListModels, it will
also help us create widgets from GListModels easily.
