FEATURE: app specific scheme

[ChristopherA]

For use with #132, for support of x-callback, in we may want to support a GordianSeedTool: scheme to more guarantee that the x-callback returns to GST. Especially if more apps start supporting UR.

This scheme should probably support both GordianSeedTool:UR:* GordianSeedTool:* as being the same.

Investigation is require to understand best practices of naming of schemes and returned URLs in x-callback.

(refactored from #124)