Refer to https://github.com/cocos2d/cocos2d-x/pull/3363

Reason:

It's common case to check whether a timeline exist by name; So CCBAnimationManager::getSequenceId should be public;

Adding the CCBAnimationManager::getSequenceDuration method to get duration of exist timeline is pretty usefully when we make our animation all with cocosbulder.

Yes, there already is a method CCBAnimationManager::getSequence(int nSequenceId), then we can call CCBSequence::getDuration to get that info. However, CCBSequence is not binded for JS, we need a more simple interface to get that info for JS.