GrabDragInteraction should factor out model for reuse

[zepumph]

From phetsims/density-buoyancy-common#368, we keep a Mass model instance around for multiple copies of the MassView (which creates the GrabDragInteraction). So let's make a grabDragInteractionModel that can be persistent through different grabDragInteraction instances. And perhaps such that multiple views can update the same model (for cue logic). We will see!

[zepumph]

I got to a commit point:

• TODO about reset (in Design logic for when to show the grab and drag cues density-buoyancy-common#368 (comment) @samreid wrote "For reset, I recommend that Mass.reset resets the given GrabDragModel (don't try to have some of them skip that step and do it at the shared declaration.)")
• What else wants to go into this model? It could be hard to share between instances if it holds the "grabbed or not" interaction state.
• Rename to be more about cue state?
• Move to a different file?
• Use Properties to be foreward thinking
• see Grab drag cue bug density-buoyancy-common#382) Bug in Density compare model: tab to last block, grab with space, shift+tab and see the grab cue still in the 3rd block. (related to bug in GrabDragInteraction: Balloon drag cue doesn't hide immediately when moving the balloon. #868 and to Property note above, maybe we need emitters!?!?)

[zepumph]

Noting that some of the above may overlap with #869. Note that issue too.

[zepumph]

All items have been checked above. Not all because they are done; many will be handled over in #869. Closing