Add ability to use the colorbar video device

Ignore-this: 44a1fca930b5cea9f4320745c1e5cae4

Disabled by default

darcs-hash:20140912153940-56b4d-4e03421cd0013c3c06389238269214ea33f646de
Handle video device changes

Ignore-this: c92b68fc0dbeaf53880a422263b79d7e

Switch back to system default (the first detected one) if
if the one in use vanishes. Currently we cannot detect this
at runtime, but tiggling the colorbar device on and of triggers
it.

darcs-hash:20140912154244-56b4d-8d1a2e97180b1b0b37f552dbbec0e0d75eb8b3d2
Update readme

Ignore-this: a6b9919af55409c8a7c37b0d6f08c536

darcs-hash:20140914020441-58252-93300af6bbd21396e88ab8105e19b896e883d5c5