Prevent vectors from being returned to invisible toolbox or invisible "slot". #341
Description
Related to #258, PhET-iO instrumentation...
The vector toolbox is configurable via PhET-iO. Individual "slots" in the toolbox can be hidden, and/or the entire toolbox can be hidden -- both of which present UX problems. Currently, the user can return a vector to an invisible toolbox (or invisible slot) by dropping the vector outside the grid (or eventually by using the keyboard). This prevents the instructional designer from creating a sim configuration with a fixed number of vectors, which we suspect will be a common use case.
I discussed with @matthew-blackman, and we think the best path forward is as follows. If the toolbox or slot that a vector would return to is invisible, that vector should be constrained to dragging within the bounds of the grid, similar to vectors in the Equations screen. And any keyboard shortcuts for returning the vector to the toolbox should be disabled for that vector.