Add 'tag_id' to node table to allow multiple unique tags per record_id

[saxondigital]

Currently there is a limitation where nodes with the same record_id and table_id must have the same tag title. However, sometimes you want multiple nodes to represent a single record, with distinct tags for these nodes, (such as with ANTz-GitViz.)

We propose to add a 'tag_id' to the node table (by re-allocating one of the existing columns.) This would allow the node to reference a specific tag while sharing the same record_id and table_id pairs. The tag table would stay the same with the first column (tag_id) being referenced by the node.

Backward compatibility would be maintained by using the existing record_id / table_id matching when a node has a tag_id = 0.

Note that the current workaround is to create tags with the same record_id but with distinct table_id's. This is a bit messy, but is compatible with the current version and would remain so with the proposed changes.

[saxondigital]

Relates to issue #151 where user generated tag record_id's (in the GUI) can possibly conflict with those loaded using a tag table.

[saxondigital]

Re-purpose 'format_id' field in np_node table to be our new 'tag_id' column.

The difference can be used to detect which table version we have.
Relates to issue #237 to detect backwards compatibility.