Refactor HtmlScript.executeEventIfNotIE

I think we have to change this method into something like
private void executeEventIfSupportsFeature(final String type, final BrowserVersionFeatures feature)
and split the feature GENERATED_9 into two (TRIGGER_ONLOAD_ON_EXTERNAL_JAVASCRIPT and TRIGGER_ERROR_ON_EXTERNAL_JAVASCRIPT.

At the moment it is a bit to complicated to create a patch, because i have to many changes open for HtmlScript (see bug 3055542)