Use a UI toolkit

[LaCuneta]

At the moment, the core NetTango block manipulation library UI is stuck together using imperative code with the Web APIs, which is fine for such a simple app. But longer-term it's probably better to use something with more support so we can build on the work of others for new features. Galapagos uses Ractive, but of course there are many other choices.

Using a toolkit would also make merging in the builder pieces that currently live in Galapagos back to core, so less of the NetTango code is directly tied up with NetLogo Web.