[1.10] Ref #13353: capture onlyHandlers in jQuery.Event.isTrigger

This is not a fix, but it does make it _possible_ to fix with a ([large](https://github.com/gibson042/jquery/compare/jquery:d79bf3517eda9b74883c68c255e82067449d3274...gibson042:13353)) plugin—at least for [#13353](http://bugs.jquery.com/ticket/13353).

I tried a [general solution](https://github.com/gibson042/jquery/compare/jquery:4083a19192069ac40eb0fe35c3f462ff1986838c...gibson042:13428) to also sweep in [#13428](http://bugs.jquery.com/ticket/13428), but unfortunately the interactions between focus and blur killed it for oldIE.