Integrate Scala.js into the XForms/Form Runner client

[ebruchez]

We crave using Scala on the client!

• Scala.js is no longer experimental as of early 2015.
• impact on JS size
  • uncompressed optimized JS for small project w/ e.g. Vector and List: ~200-300 KB
  • compressed: ~70 KB
  • would add ~100 KB for moderate code size on the client
  • could offset some of this by removing some legacy libraries (see Progressively get rid of YUI #1599, Remove YUI RTE and map HTML textarea to tinyMCE #1249, etc.)
• impact on development
  • right now compiled w/ sbt
  • reloads in browser not as instant as with JS/CoffeeScript
  • roundtrip for very small project ~5 seconds to reload w/ Chrome dev tools
• impact on build system
  • need to figure that out, but build system needs modernization anyway

[ebruchez]

The current thinking is to start with Form Builder, which:

• requires more complex client-side code
• has higher browser requirements
• is often installed in controlled environments

Therefore, adding JavaScript code shouldn't be an issue.

Candidate features to investigate:

• Ability to move controls with drag and drop Ability to move controls with drag and drop #1479

[avernet]

I'm tempted to schedule making some progress on this for 4.8, if only because working on this is pretty exciting. (Is that a good or a bad excuse?)