feature: integrated model-controller widget with setup, go, go-once

[SethTisue]

nearly any standard model has or should have three standard buttons:

• setup
• go forever
• go once

NetLogo doesn't give you any help making these buttons. It should.

we have sometimes called this feature "VCR controls" or "YouTube controls" since we might decide to use little "play" and "stop" icons (that come from VCRs, and before them, tape decks) instead of (or in addition to) the words "setup" and "go".

the lack of this feature is a usability problem for model users, since the standard model controls vary so much from model to model in appearance, layout, and naming. we shouldn't force users to have to puzzle out custom controls every time for standard operations.

and it's a usability problem for model authors, because it's tedious and error-prone to have to make the same buttons over and over again, and because authors make these common mistakes over and over again:

• failure to include a "go once" button at all
• failure to check the "Disable until ticks start" checkbox for "go" and "go once"

other simulation and programming-for-kids tools typically provide an integrated, standardized model controller widget. looking at what Scratch, Repast, and others do would probably help us with a design.