Allow using a Function as the cancel option instead of a Selector.

ui/widgets/mouse.js

@@ -90,9 +90,11 @@
 
 		var that = this,
 			btnIsLeft = event.which === 1,
-			elIsCancel = typeof this.options.cancel === "string" ?
-				$( event.target ).closest( this.options.cancel ).length :
-				false;
+			elIsCancel = typeof this.options.cancel === "function" ?
+                                this.options.cancel.call(event.target, event) :
+                                ( typeof this.options.cancel === "string" ?
+				  $( event.target ).closest( this.options.cancel ).length :
+				  false);
 		if ( !btnIsLeft || elIsCancel || !this._mouseCapture( event ) ) {
 			return true;
 		}