* [ASVideoNode] Add delegate method called when the currentItem is set.
* [ASVideoNode] Add videoComposition and audioMix properties to the ASVideoNode.
* [ASVideoPlayerNode] Add new initialiser with videoComposition and audioMix, and forward new delegate method.
* [ASVideoPlayerNode] Forward missing ASVideoNodeDelegate methods.
* Removed play button from ASVideoNode
* Removed play button tests
* added some checks in ASVideoPlayerNode
* added new delegate method for time formats
* changed static string to ASVideoPlayerNodeControlType
* public property for duration
* public method for seek
* new _controlFlexGrowSpacerSpec; used to use flexible space in
controlbar. for example if user does not want to have scrubber, he can
replace it with flexible spacer and duration text node will be kept at
the right edge.