Ability to clear a fraction shape?

[KatieWoe]

For phetsims/qa#250, phetsims/qa#252, and phetsims/qa#253
When building a shape you can either hit the undo button several times or drag the shape into the bin and drag it out again if you want to start over. It may be useful to have a way to clear the shape of pieces, but still leave it up. I'm not sure if there is an elegant way to do this though.

[amanda-phet]

Since you can drag the shape to the panel I think that is adequate. I don't like the idea of adding another control to the container (such as a reset), but am open to considering something else. @ariel-phet @kathy-phet ideas?

[ariel-phet]

I am going to mark this question deferred for the moment. Certainly for 1.0 I do not think we want to mess with this behavior