Fix widget id being unchangeable

While not recommended, the widget id should be able to be changed. And
it definitely should not throw non-self-explanatory error messages.

Change-Id: Ic2cdbde3d201e2af52616dea1be941d61452fd52

doc/api/Widget.json

@@ -167,7 +167,7 @@
   "properties": {
     "id": {
       "type": "string",
-      "description": "A string to identify the widget by using selectors. IDs are optional. It is strongly recommended that they are unique within a page."
+      "description": "A string to identify the widget by using selectors. IDs are optional. It is strongly recommended that they are unique within any component."
     },
     "class": {
       "type": "string",

src/tabris/Widget.js

@@ -227,7 +227,10 @@ export default class Widget extends NativeObject {
   set id(value) {
     /** @type {string} */
     const id = types.string.convert(value);
-    Object.defineProperty(this, '_id', {enumerable: false, writable: false, value: id});
+    if (id === this._id) {
+      return;
+    }
+    Object.defineProperty(this, '_id', {enumerable: false, writable: true, value: id});
   }
 
   get id() {

test/tabris/widgets/Widget.test.js

@@ -239,10 +239,11 @@ describe('Widget', function() {
       expect(widget.id).to.equal('foo');
     });
 
-    it('gets id property from widget.id', function() {
+    it('can set id twice', function() {
       widget.id = 'foo';
+      widget.id = 'bar';
 
-      expect(widget.id).to.equal('foo');
+      expect(widget.id).to.equal('bar');
     });
 
     it('stores class property in widget.classList', function() {