Add video annotation support

This work is still work in progress, doesn't seem to work for http://acroeng.adobe.com/Test_Files/classic_multimedia//VolvoS40V50-Full.pdf

It does seem like it worked in the beginning, but doesn't seem to work for me.  

Rebasing change from pull request #4376 to get some reviews and pitfalls. 
Why do we rule out `"maybe"`? If there is no plugin that handles the MIME-type, then we're happy to try to embed the video when `checkSupport === 'maybe'`.