Allow custom event handlers for the click event of toolbar buttons

Example:
mw.toolbar.addButton( {
	imageFile: '//upload.wikimedia.org/wikipedia/commons/a/a9/Button_tournesol.png',
	onClick: function(){ alert( 'MediaWiki!' ); }
} );

Change-Id: I615960f689a0f8d35a12879efebc0afda8eef7b1
Merge "Allow custom event handlers for the click event of toolbar buttons"