Implemented wikibase toolbarlabel and toolbarbutton widget

The change set removes some of the legacy JavaScript code by replacing
wikibase.ui.Toolbar.Label and wikibase.ui.Toolbar.Button with
jQuery.wikibase.toolbarlabel and jQuery.wikibase.toolbarbutton. For
consistency, from now on, a button is always represented by an <a> tag
- enabled/disabled state is managed via css styling and JavaScript,
tabbing is suppressed by assigning a negative tabindex attribute.

Change-Id: I8f61a767ae6cfc369bd04dc34d902d0dc2736109
Merge "Implemented wikibase toolbarlabel and toolbarbutton widget"