support backend-driven pulseaudio property setting

- pulstream: new property role (== PA's media.role)
- pulsesupport: new func streamproperties
  returns a hash of per-stream properties a backend can set
- pulsesupport: new func setupstreamenvironment
  can be used by a backend not supporting per-stream properties to set
  the properties to the right value as close to stream creation as possible
  to reduce the scope of 'reentrancy' issues, howeve if two MO's are
  started right after one another the second AO's envrionment will still
  shadow the first's

app_name/version/icon_name are retained as always-on override properties
as we consider them static and therefore backend driven setting isn't
really necessary (also not possible with VLC<2.1)

when using pvlc (which uses the setup function due to lack of property
support through libvlc) a warning will be displayed in debug mode to
make clear what could happen.

CCBUG: 321288
FIXED-IN: 4.7.0