Improve media timeline creation

When coming from a live timeline (or from a pinned timeline), it should be possible to create media timeline using `Live` so that the media event does not have to be loaded again.

Currently we are using a detached media timeline in all cases, except if the gallery has been opened and the media gallery timeline knows the event.

EXI is already using this.

Note: the media timeline may not contain the event immediatly, the current mechanism to display the cashed data app side should be preserved.