Add support for creating wiki links with auto-generated label

The wiki syntax supports links with auto-generated label:

[[Path.To.Page]]

The way the label is generated can be configured in xwiki.properties. It can be the page title, the page name, etc.

The CKEditor preserves the links with auto-generated label, as long as you don't modify the (generated) label, but it doesn't offer support for creating such links. An idea is to create links with auto-generated label when the user doesn't specify any label.

The advantage of these links is that they behave better when you rename or move the linked page. The label being auto-generated is going to be kept in sync.