Plugin popup() api

Changes to JS API:
- popups have backdrop and is centered
- plugin.popup() takes a string custom element name
- plugin.popup() returns an API for closing and reopening the popup
- plugin.deprecated.popup() takes Element (similar to GWT Plugin JS API)

Recommended usage:

``` js
  Gerrit.install(function(plugin) {
    const popup = plugin.popup('my-plugin-popup-simple');;
    // ... work
    popup.close();
    // ... more work
    popup.open();
  });

```

``` html
<dom-module id="my-plugin-popup-simple">
  <template>
    <div>popup popup popup popup popup </div>
  </template>
  <script>
    Polymer({is: 'my-plugin-popup-simple'});
  </script>
</dom-module>
```

Change-Id: Icb3f83d35f3c60915f12b77bc8a7d548d50d5695

polygerrit-ui/app/template_test_srcs/template_test.js

@@ -27,13 +27,15 @@ const EXTERN_NAMES = [
   'GrGerritAuth',
   'GrLinkTextParser',
   'GrPluginEndpoints',
+  'GrPopupInterface',
   'GrRangeNormalizer',
   'GrReporting',
   'GrReviewerUpdatesParser',
   'GrThemeApi',
   'moment',
   'page',
-  'util'];
+  'util',
+];
 
 fs.readdir('./polygerrit-ui/temp/behaviors/', (err, data) => {
   if (err) {
@@ -102,4 +104,4 @@ fs.readdir('./polygerrit-ui/temp/behaviors/', (err, data) => {
       process.exit(1);
     }
   });
-});
+});