SpriteFrameCache load from plist file content data

add

SpriteFrameCache::addSpriteFramesWithFileContent()
SpriteFrameCache::removeSpriteFramesFromFileContent()

this is a fix for #7566

how to add test case?
Would you please change `plist_content ` to `plistContent ` to obey cocos2d-x coding style?
Thanks.