Issue_151 Improve GetLocalisedXGMML

Dedup common edges between subgraphs.
Improve "0" level support.
Ensure top level self contained clusters are included.
Ensure message text is always visible.
Caching of SVG ignored externally merged SVG.

Fixes Issue_151

Signed-off-by: Gordon Smith <gordon.smith@lexisnexis.com>
Merge pull request #153 from GordonSmith/Issue_151

Issue_151 Improve GetLocalisedXGMML

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>