Listbox improvements

• Add overflow scrim

• Allow centering custom list item checkbox
• Fix rendering of very long labels - related dropdown issue ??? - this does not seem to be an issue any more
• Improve selection performance. More details
• clicking on elements outside the listbox that stop click event propagation will leave listbox open. Most likely have to be fixed in the v-ouside directive implementation. More details here - will be handled in scope of a different issue #1881 (closed)
• Add separate event to emit full selected object instead of value only