In mod_conference 1.6.x, all conference users could get the audio floor
regardless of the kind of video layer they were placed in.

In 1.8.x, the concept of a 'dedicated video layer' was introduced, such that
video layout slots with a reservation ID or a role ID are marked with the
'MFLAG_DED_VID_LAYER' member flag when a member is in the slot, and such
marked layers are prevented from becoming the conference audio/video floor
holder.

This makes sense for the video floor in all circumstances, and the audio floor
in most circumstances.

However, there are some circumstances where allowing users in these dedicated
video layers to become the conference audio floor holder is preferred, such as
when all slots in the layout have reservation IDs, and the audio floor events
are used to manage users in those slots manually.

This adds a conference profile flag, 'ded-vid-layer-audio-floor, which, when
enabled, allows conference members in dedicated video layers to become the
conference audio floor holder.