Make 'scrubbing' an official property

[heff]

As a follow up to #1741, scrubbing should become an official player API property. Currently the progress bar sets player.scrubbing = true, and if we change that now it could break external control bars, so this should wait for 5.0.

Here's the code to make it happen. The progress bar will need to be updated.

/**
 * Store the internal state of scrubbing
 * @private
 * @return {Boolean} True if the user is scrubbing
 */
vjs.Player.prototype.scrubbing_ = false;


/**
 * Returns whether or not the user is "scrubbing". Scrubbing is when the user 
 * has clicked the progress bar handle and is dragging it along the progress bar.
 * @param  {Boolean} isScrubbing   True/false the user is scrubbing
 * @return {Boolean}               The scrubbing status when getting
 * @return {Object}                The player when setting
 */
vjs.Player.prototype.scrubbing = function(isScrubbing){
  if (isScrubbing !== undefined) {
    this.scrubbing_ = !!isScrubbing;

    if (isScrubbing) {
      this.addClass('vjs-scrubbing');
    } else {
      this.removeClass('vjs-scrubbing');
    }

    return this; 
  }

  return this.scrubbing_;
};

[forbesjo]

Looking at this

[heff]

Closed by #2080