Allow buttons to take tasks as commands #154
Description
This was made possible by @qiemem's work on #149.
It would be worth considering replacing the commands property by something like an on-click event, for coherence.
As mentioned on #149 (comment) :
One caveat is that xw:export/xw:import won't work for tasks (NetLogo/NetLogo#22), but I think we can live with that if its documented.