Combine DragListener and KeyboardDragListener into a single class

[zepumph]

While working on phetsims/scenery-phet#849, we found that RichDragListener and RichKeyboardListener are identical, but need to be duplicated because they DragListener and KeyboardDragListener are separate classes. I wonder if we could create a third class that combines them into a single drag listener, and have RichDragListener extend from that. @jessegreenberg @pixelzoom, I couldn't find another issue for this, but please close as a duplicate if I missed it. I know we have discussed this before.

[pixelzoom]

I also thought there was a similar issue, but can't locate it. It's unfortunate that we need to add both a DragListener and a KeyboardDragListener, but I'll be surprised if we find a way around that.

[jessegreenberg]

This is probably a subset of #1238.

I wonder if we could create a third class that combines them into a single drag listener,

Great idea!

[zepumph]

I also was teasing with the idea of doing it in a different way, where we make KeyboardDragListener a subtype of DragListener. Perhaps calling it FullDragListener. And it may not be too challenging to handle an "off switch" for each type of input for backwards compatibility and complicated cases. No matter, this isn't the kind of work we could just sprinkle into an iteration.

[zepumph]

Over in phetsims/scenery-phet#849, we need KeyboardDragListener to have an interrupted flag like PressListener. I'll add it here.

[zepumph]

This was also nice, as it aligned the API for the end callbacks.

[jessegreenberg]

I am trying out an idea for this in branch richDragListener_scenery_1614. It simply creates both a DragListener and a KeyboardDragListener and forwards the input events to each. Options are set up so that many are shared by the two listeners but you can still provide options to each listener as needed.

[jessegreenberg]

A combined RichDragListener was added to scenery-phet using the approach from #1614 (comment). It is in scenery-phet because it combines the "rich" drag listeners that also include tambo sounds. Ill prune the scenery branch now that we are done with it and transfer this issue to scenery-phet to wrap up.

[jessegreenberg]

Code review will occur in phetsims/scenery-phet#858, closing.