Move UILableImpl as TNSLabel in widgets. (#2298)

Refactor Label class to the new use TNSLabel class.
Added definitions.

tns-core-modules/org.nativescript.widgets.d.ts

@@ -177,4 +177,9 @@
             }
         }
     }
+}
+
+declare class TNSLabel extends UILabel {
+	borderThickness: UIEdgeInsets;
+	padding: UIEdgeInsets;
 }