Reverted back to internal screen sharing handler variants

Ignore-this: ef50ac8589d12e0a301e454c8e6d014

darcs-hash:20140424104312-b22ec-36bac7f6f4c4b93db3bd84dacc1af20c93a693fd
Add support for adding/removing multiple streams at once

Ignore-this: d8ab7a8a27aefa32dbe7eb942a2c8696

darcs-hash:20140425123035-56b4d-ee614dd19ec2888b1b70759aed820eecd0132b74
Fix closing media transport to avoid leaking STUN sockets

Ignore-this: b05f6d3d300e58c5d35c65f292db6308

darcs-hash:20140515105750-56b4d-71c8108bb443d630097db4ea7beb2be46d12c2fe