Implemented toolbar and toolbareditgroup jQuery widgets

Getting rid of some "old UI" components (wikibase.ui.Toolbar, wikibase.ui.Toolbar.Group,
wikibase.ui.Toolbar.EditGroup) by implementing jQuery.wikibase.toolbar and
jQuery.wikibase.toolbareditgroup. A dedicated toolbar group class/widget has become
obsolete by using cascaded toolbar widgets.

Change-Id: I52160f2ccb098c2c00511243fc9d7b7b8f399d1a
Limiting number of registered event handlers in toolbar button widget

Change-Id: I111e61f2505da49598a854b034bae3055271aba1
Removed toolbarlabel's get/setContent method

There is no need to have dedicated get/setContent methods. After converting to a jQuery
widget, instead of using dedicated methods, jQuery's native contents() method can be used.

Change-Id: I21c279ccacfb27d2f47ef98fffc02a6d022630de
Merge changes I21c279cc,I111e61f2,I52160f2c

* changes:
  Removed toolbarlabel's get/setContent method
  Limiting number of registered event handlers in toolbar button widget
  Implemented toolbar and toolbareditgroup jQuery widgets