[Parts] Fix part including overrides in content

The part binding handler was incorrectly including part overrides in
the 'content' part when used by widgets
[Expandable] Footer part + open class on element
[Expandable] Extend, not replace, binding context
[Perf] Register global event handler for actions

2 document-level event handlers are used to handle all 'action'
binding handlers to avoid registering a lot of event handles,
particularly
in the case of a foreach loop. Can save hundreds of event handlers being
registered, automatically.