Expose Document.isTranslation() to the web API

From the web API it is not that easy to find out if the Document instance you are holding is the main document or a translation.

Such situations can occur when using:

- $doc.getTranslatedDocument()
- $xwiki.getDeletedDocument(...).getDocument()
- etc...

The new method should look something like this:

$doc.isTranslation()