Show signals in RST visualizer

[lgessler]

(Note: for your convenience I'm running a demo server for this PR. Please send me an email for the address, I'd prefer not to post it publicly.)

This PR adds support for signals to the RST visualizer. In RST, "signals" are pieces of evidence, such as a telling word, that act as cues for a reader/listener as they attempt to find the appropriate relation between two discourse units.

This extension parallels the visual approach used when signal support was added to rstWeb, and assumes the presence of Salt structures like those produced by the RST Pepper module, which in turn can be produced by exporting .RS4 XML from rstWeb and put through Pepper.

A sample screenshot:

[lgessler]

(Added the WIP tag, but I changed my mind. @amir-zeldes and I are happy with this and think it's ready to merge unless you think there's something else we should change.)

[thomaskrause]

Thank you very much for the PR, this looks good and I merged it into develop. The next feature release will probably be already with graphANNIS as a backend (and with Version 4.0). If you need the PostgreSQL version to support this visualizer as well, we might need to think about back-porting it and making an ANNIS 3.x release.

[lgessler]

@amir-zeldes FYI

[amir-zeldes]

I'm fine with not back porting it, since 4.0 should completely replace 3.X and is much better in so many ways. We may want to build a 4.0 snapshot kickstarter with the new visualizer to demo at NAACL though. Is there an auto build kickstarter somewhere that would include the updated visualizer?

[thomaskrause]

I just uploaded the first beta version of ANNIS 4 which can be downloaded from http://corpus-tools.org/annis/download.html 🎉

This will surely not be the last beta until we release a stable ANNIS 4, but I think it is a good start and easier to refer to in comparison to auto-builds 😃