Fix Live CSS with an iframe element

For #7935 and #7785 

Bugs to fix:
- [x] Only updating the iframe if the same CSS file is used for both iframe and page
- [ ] Changes to the iframe's HTML will only take effect on save
- [ ] Error message on startup (in console)
Nope, it can have multiple values (that's what this PR is all about - we assumed it can only have one value before), but they should all be the same, so it doesn't matter much which one to use. And as they are enumerated (but __not__ beginning with 0), the first one should be ok...