Update text track management
Created by: nicosang
Hi,
this PR has to not always create a new text track on the html5 video element when a new stream is loaded. Unfortunately, text track can't be removed. The idea is, when a creation is asked, to detect if this kind of track (same label, same kind, etc....) has been already created. If it's true, this track is returned, otherwise a new track is really created. does it solve issues #1294 (closed), #1394 (closed)? @davemevans if you can take a look......thanks! ;-)
Nico