Added support for full database rebuild (from File menu).
If you were using previous versions of TkGutenBrowse, you
must do this to access image and audio files.
Prior to this release, the URL's for non-text files were
just plain wrong (my fault, not Project Gutenberg's).
Updating the database adds new entries,
but does not fix old ones. Rebuilding the database takes a
little longer, but will fix the old data as well as adding
new entries.