plugin.getDomHook now returns a DOM Hook API instance that can be used
to setup callbacks for the dom hook insertion into DOM:
``` js
Gerrit.install(plugin => {
plugin.getDomHook('reply-text').onAttached(element => {
if (!element.content) { return; }
element.content.style.border = '1px red solid';
});
});
```
Includes tests, and refactoring for existing plugin api
and internal endpoints data model.
Documentation coming in next change.
Change-Id: Iefa0cac4257323fc4557dfb7c196a4a81e001eea
d559f19f1f