Refactored toolbars

Simplified the toolbar mechanisms and optimized the toolbar widgets in terms of modularity.
Removed $.wikibase.toolbarbase, $.wikibase.toolbareditgroup and $.wikibase.toolbarlabel.
Implemented $.wikibase.toolbaritem which represents the common base constructor for every toolbar
item.
All toolbar widgets inherit from $.ui.TemplatedWidget now achieving consistency with the other
$.wikibase.* widgets.
disabled/enabled state handling now uses native jQuery widget functionality.

Deploy: This requires bumping the parser cache epoch.
Change-Id: I00ca6dd6a62972032e12bf84b3e0bf9b64d0678a