Rework code path for outgoing calls

See the opening post in !301 (merged)

mentioned in merge request !301 (merged)

Basically this means not always falling back to use the selected origin from the HdyComboRow (which is a reasonable thing to do when the outgoing call is placed from the dialpad, but not so much when the call is initiated by f.e. dialing from the contacts).

changed milestone to %Basic SIP functionality

Needs #194 (closed) closed first (aka !301 (merged))

We should prompt the user (see the designs https://gitlab.gnome.org/Teams/Design/app-mockups/-/blob/master/calls/dialing.png) when not falling back to a "default" origin (which then would need to have a setting in the preferences).

Relevant insight: !301 (comment 155209)

!301 (comment 155205)

In fact wouldn't it be even nicer if the provider itself would provide the GListStore interface instead of having that provied via _get_origins ()? But that would likely be an improvement for the future.

Would be nice to have in this issue

Somewhat related: #293