OSMF SVN

* Introducting ContentDownloadableTrait.
* Adding bytesLoaded and bytesTotal getters to ContentLoader. The latter fires a BytesTotalChange event, that ContentDownloadableTrait listens to.
* Updating ContentElement to instantiate the new trait.

Extending Example player:

* Adding 'bytesDownloaded:' and 'bytesTotal' fields to the UI.
* Hiding/showing the fields according to the Downloadable trait being available or not.
* Added plumbing to update the values.

Changes to previous commits:

* DownloadableTrait: CTOR: removed 'target', for we want the trait to always be the dispatcher of its own events.
* SoundDownloadableTrait: made class internal, removed 'target' from CTOR, added listener for Sound's OPEN event to ensure getting 'bytesTotal', and changing events to be dispatched on changing the value from/to NaN.
* BytesTotalChangeEvent: added 'clone' method.
* MediaPlayer: added code to start the downloadable timer on the Downloadble trait being added/removed. At 'onDownloadTimer', now also dispatching changes from/to NaN.
* BytesDownloadEvent: renamed to BytesDownloadEventChange, and made it about bytes downloaded only, using our newValue/oldValue convention. Added 'clone' method.