(bug 44538) jQuery.wikibase.listview.ListItemAdapter value awareness

* new option 'listItemWidgetValueAccessor'
* null instead of undefined for 'empty' values. undefined will just fall back to null in functions
  where the value is optional
* includes some general improvement and documentation updates
* adjusted statementview to work with the changes

Change-Id: I6fec47cc85da9f17a76c58ace0f1b04852a7d611
Merge "(bug 44538) jQuery.wikibase.listview.ListItemAdapter value awareness"