Guest frames pass an identifier for their associated browser frame to the
sidebar. The sidebar in turn uses this identifier as a key to refer to different
guests in various places. This identifier used to be sent as part of the
`documentInfoChanged` message after a guest <-> sidebar connection is
established, rather than as part of the initial connection metadata. As a result
the sidebar had to invent a temporary ID for the new guest, which was used until
it learned the real ID.

This commit simplifies the picture by adding a `sourceId` attribute to frame
connection setup messages. This replaces the `frameIdentifier` in the
`documentInfoChanged` message, and allows the sidebar to know the final ID of
the guest as soon as it connects.