dropper: disable dragging without disabling button

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Reported by @kathy-phet in #118 (comment):

it is not currently possible to make it unmoveable/unpickable without also making the add liquid button unpickable/not working.

This is dependent on completion of phetsims/scenery#1041.

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In phetsims/scenery#1041 (comment) and phetsims/scenery#1041 (comment), @kathy-phet said that this feature is not a prerequisite to delivering a dev version (due 3/30/2020), and only becomes blocking for the final release (date TBD) if identified by the client.

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... only becomes blocking for the final release (date TBD) if identified by the client.

The client did not flag this issue in their feedback. @arouinfar @kathy-phet is it still the case that this does NOT need to be addressed for 1.0 publication?

[kathy-phet]

I think for 1.0 published release we do not need to address this issue.

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Another way to address this issue would be to attach the DragListener to a subset of the dropper's scenegraph (excluding the button). That would require changing the EyeDropperNode API, and probably changing its internal structure. And then clients would have to use a non-standard way of adding a DragListener to an EyeDropperNode.

Imo, coming up with a way to disable scenery input listeners is still the better approach. Using pickableProperty will always have this general problem, and it's not a great way of disabling interaction, because it forces us to have a pickableProperty for every Node, regardless of whether it's interactive.

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This was addressed in #215, closing.