This avoids confusion when only the SIP plugin is loaded, but no account
is being used.
For this we introduce two new functions in the abstract CallsProvider class:
`is_modem()` to indicate whether the the provider deals with PTSN telephony and
`is_operational()` which by default simply checks if there any avalaible origins
These are the protocols that the provider plugin may support. The origins
must be queried independently whether or not they support any given protocol.
Example: A SIP origin/account may or may not support the "tel" protocol.
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.
Modems being added or removed were not working. To fix this, we pay
attention to the "object-removed" event and not just
"interface-removed".
Also, to deal with ModemManager appearing and vanishing, we add a
GDBus watch on ModemManager's D-Bus object.
Finally, we provide appropriate UI feedback when it's not possible to
make a call.
Closes#15Closes#16
