Only stop if the transition on this element ended

This way it doesn't end when some random element elsewhere on the page has an ending transition.

It must be noted that this is not a complete fix. If you have multiple transitions on the element, it still ends when the first ends.

I ran into this not working (the transition ends almost immediately, because some unrelated transition ends). This fixes it for me, but I was wondering whether this cornercase was considered, and whether the other cornercase (multiple transitions) was considered.

I think it's very hard to get this correct, and think it might be better to make the timeout explicit, or at least allow to set it explicit, and fall back to this behaviour. But I'm really interested in feedback on this one.
Added event parameter
Make sure it works when we have no css transitions
add picture element and related attributes
add picture to xjs.knownTags
Add support for <dialog> tag.

fixes #2017
Throw an error when functions on `statics` clash due to duplicate keys
Throw if a reserved property is defined in 'statics'
Deprecate 'return false' in event handlers
Re-add  with warning and monitorCodeUse
Fixed style and comment nits
Test for both true and false
More info in monitorCodeUse call
Look on __reactBoundContext for displayName
Add 2048-react to open-source demos

Nothing too complicated, but I think it could be a fun beginner demo.
Indentation for eventHandlerName
use attribute for media, srcSet and sizes
Including missing validation check

The validation check was removed from the update to the CommentForm handleSubmit function.
Removed monitorCodeUse call
Adding `e.preventDefault()` to `handleSubmit`, 

Also added plain `return`, updated text.