Simplify tool button a little

No need for a list of image layers, as these aren't being
created dynamically.

No need to catch bad button types. The error thrown by the state
chart will have the same effect as the console.warn message (and
the author will have created a blank button, so it should already
be pretty clear).
Allow agents to render in env in another context
Add carry contexts to plant species
Add more mouse events to the env view
Create carry tool
Add carry tool to demo