Fix some issues with star rating code

Fix some issues with star rating code

To waste less space in the tree view heading we simply put a star in the
heading instead of "Rating".

We now treat "zero stars" to mean "not rated" and don't store that value
in the XML file.

Rating is no longer a top level tag in the dive entry but instead a
property of the dive tag.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>