Different icons for HTML5 video

Resolves #1522.

Built and tested in Chrome, no obviously apparent bugs.

I do have a few concerns about how I did this, see the comments on the diff.
Originally I tried to add a new elem.type (at around line 680) but the expando wouldn't open (I'm assuming that it had something to do with expando open/close handling) - is it acceptable to do it this way instead?