Using Objective-C attributes, in this case `unavailable`, we can hide
unsupported APIs at compile time instead of detecting and warn about it
at runtime with a set of asserts.
* [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.
* [ASVideoNode] Change superclass to ASNetworkImageNode so that ASVideoNode can be its own placeholder
- remove _placeholderImageNode property of ASVideoNode (use self.image now instead)
- move layoutSpecThatFits: code to calculateSizeThatFits: & layout: methods as ASImageNode uses calculateSizeThatFits:
* [ASVideoNode] Tweaks to the definition of the delegate protocols to integrate with ASNetworkImageNode (superclass)
* 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.
Unclear why the framework target required these seemingly unrelated changes to Collection
classes, but I modeled them after related files and it seems cleaner now (passes tests & framework target)
