Prevent changing of playback speed while playing live content

[sp1ritCS]

Consider force setting the playback speed to 1.00 and setting the sensitive property of the playback slider to false for "live' content. Right now, if you try to increase the playback speed clapper will crash, as it tries to access part of stdin which doesn't exist yet.

[Rafostar]

In addition to the player, seems like something that should have an assertion in the API, thus adding API label.

[Rafostar]

Note to self: live content should ignore playback speed value and not change/restore it to/from saved preferences.