Call move support

This adds support for the [Call Move feature](https://trello.com/c/qxuepdTn/315-2-as-a-user-i-can-move-to-another-device-if-the-spa-supports-it).

It allows an SPA to expose [capabilities](https://wiki.mozilla.org/Talkilla/SPA_API#Capabilities), here especially the `move` one.

Right now there's no graphical button but only a text link in the call control bar for moving a call using an SPA which supports it (warning, the default Talkilla SPA doesn't support it).

Clicking on the *Move Call* link will post a `talkilla.initiate-move` event to the worker, which forwards it to the SPA as an `initiate-move` one, with the following data attributes passed:

- `callid`: the call id
- `peer`: the peer identifier

It's obviously up to the SPA implementer to actually process the call moving operation.

This work has been pair programmed & reviewed by @ametaireau and @n1k0.