FR: Allow user to generate, save, and use saved EXI grammar to encode/decode file

[jsbiff]

Right now, exificient-gui seems to always need to generate grammars from an XSD file. This process seems to be a little bit slow, and xsd's don't typically change very frequently. Also, right now, if one wants to save an EXI grammar, you must separately use the exificient-grammars project.

It would be very convenient if exificient-gui allowed you to select an XSD, and generate a grammar, in any of the supported formats - json, xml, exi, or java source formats.

Then, when encoding/decoding files, add additional choice - right now you can choose schema-less, types-only, or schema file - allow additional options to choose a previously generated grammar in json, xml, or exi formats, for encoding or decoding.

[danielpeintner]

The EXIficient GUI is not a fully fletched UI like UIs for ZIP or so.
It was meant to offer an easy way to play with the EXI format.

Having said that, we do not plan to enhance much with that regard.