Use MarkerType for marker classes

Ignore-this: 1023e69a1b357cec8fa598863534b3d7

darcs-hash:20140423165651-b22ec-75d2ce321d35a6e05ea01ee0b859ca48a536088e
Removed obsolete attribute

Ignore-this: 147e70ad178f480d5f9b75cd6b404e23

darcs-hash:20140423171115-b22ec-d3315ce9a084876ea860e27de72f74467ba6e715
Refactored API for creating screen sharing streams and handlers

Ignore-this: ef0e8876de3b0d9aef3a3e444cffb7ad

darcs-hash:20140424104214-b22ec-0ae727baa0fbd566c3846427a8e06c4b2d82bd3d