Fix bookmark import to work with new layer persistence

[mdehoog]

When importing bookmarks on one computer from a bookmarks XML file exported on another computer, the layer ids will not match.

The bookmark layer implementation needs to be updated to use the new layer persistence mechanism (which no longer uses a URI for unique layer identification and creation), so that layers can be re-created when a bookmark is applied.

Perhaps, when applying a bookmark that has a layer with an id that isn't in the layer model, some kind of hash (eg MD5) of the persisted layer XML can be generated for each layer in the layer model and the bookmark, and the bookmark layers can be matched up that way.