Allow to set default origins per protocol
Somewhat similar to #406 in nature:
We should allow setting default origins to be used for outgoing calls, so that we can avoid prompting (as suggested in #406).
We already have a GSetting 'auto-use-default-origins' but that
- defaults to the first suitable origin (so this depends on the order of the added origins)
- was mostly a hack to push having to deal with #406 to the future