Eliminated wikibase.utilities.ui.StatableObject

Since the increased complexity of wikibase.utilities.ui.StatableObject is not required anymore,
the functionality is replaced by native jQuery.Widget handling.
Removing wikibase.utilities.ui.StatableObject renders wikibase.utilities.newExtension useless.
Consequently, the component is removed as well.

Change-Id: I0ff5a3949d3e5218e54d53e6a2c19d465f30eb86
Merge "Eliminated wikibase.utilities.ui.StatableObject"