Implement #42 -- DB Metadata, Group UUID

This commit implements a Metadata field for Database, whose only
member is 'recyclebin_uuid' (String).

It also implements a `uuid` field for Group, that can be matched with
the database's `meta.recyclebin_uuid`

src/parse/kdbx3.rs

@@ -186,7 +186,6 @@ pub(crate) fn parse(data: &[u8], key_elements: &[Vec<u8>]) -> Result<Database> {
     for block_buffer in xml_blocks {
         let (block_group, _meta) =
             xml_parse::parse_xml_block(&block_buffer, &mut *inner_decryptor)?;
-        // FIXME: This should only be done when _meta has a meaningful value
         meta = _meta;
         root.children.push(Node::Group(block_group));
     }