Support stream role based case handling

[ph3-der-loewe]

roard should support stream role based handling of different cases based on some kind of lists.

For example to mute or pause streams of role BACKGROUND_MUSIC when a stream with MUSIC is connected.

[ph3-der-loewe]

The current idea is to have a so called RoleStack?:
It's a stack of roles and actions like this:

Role	Action
phone	pause
music	kick
background_music	mix

if a stream of a role A connects all streams with a role below the current one will be subject to the action listed for role A.

In the above example this means if a stream of role music connects all streams of role background_music get kicked. If a stream of role phone is connected all streams of role music and background_music are paused.

The following basic actions should be defined:

mix

The default. Means all streams just get mixed as you expect.

kick

All streams below the current role in stack get kicked.

mute

All streams below the current role in stack get muted.

pause

All streams below the current role in stack get paused.

Other actions like crossfade can be introduced later.

This does not require anything on-wire (protocol change) as it is clearly only implementation domain.

[ph3-der-loewe]

if a stream connects of role B with B lower in the stack than the current one (A) it is also subject to current's (A's) action.

[ph3-der-loewe]

Was a very bad one. hope it is fixed now.