Allow import/export of the interface

[nicolaspayette]

A couple of things to keep in mind, when loading values from JSON:

• what if KIND is not defined?
• what if TAB is not defined?

[nicolaspayette]

I will need to make sure that tabs are loaded first.

[nicolaspayette]

• [x] Support import/export of Nobody (needed at least for choosers)

[nicolaspayette]

Can I support import/export of agents and agentsets? Choosers, for instance, currently allow those to used as items.

[nicolaspayette]

One thing is for sure: tasks importing won't work (basically the same issue as https://github.com/NetLogo/NetLogo/issues/22).

Agents/agentsets could perhaps be made to work, but I don't expect there will time for that. These limitations should be documented.

[nicolaspayette]

• [x] make sure that colors are exported correctly