luci-base: ui.js: implement AbstractElement.isChanged()

The new `isChanged()` utility functions allows to query the dirty state
of an ui input widget.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

modules/luci-base/htdocs/luci-static/resources/ui.js

@@ -127,6 +127,21 @@ var UIElement = baseclass.extend(/** @lends LuCI.ui.AbstractElement.prototype */
 		}
 	},
 
+	/**
+	 * Check whether the input value was altered by the user.
+	 *
+	 * @instance
+	 * @memberof LuCI.ui.AbstractElement
+	 * @returns {boolean}
+	 * Returns `true` if the input value has been altered by the user or
+	 * `false` if it is unchaged. Note that if the user modifies the initial
+	 * value and changes it back to the original state, it is still reported
+	 * as changed.
+	 */
+	isChanged: function() {
+		return (this.node ? this.node.getAttribute('data-changed') : null) == 'true';
+	},
+
 	/**
 	 * Check whether the current input value is valid.
 	 *