RNA Polymerase can be manipulated on the second screen

[phet-steele]

@aadish, on current master (3/22, noon), the RNAP on the second screen can be dragged if it satisfies two conditions:

• the RNAP has participated in transcription
• the RNAP is to the left of the transcription start site.

Once it is dragged the first time, it can always be dragged. You may need to let some transcription occur to have RNAP appear on the left.

Seen on macOS 10.12.3 Chrome. For phetsims/tasks/issues/776.

[phet-steele]

@aadish there is still a brief moment (following the steps above) where RNAP can be grabbed just before it begins transcription. I showed @aadish in person since recording this on a video is tricky.

[jbphet]

Assigning to myself since I'm taking over this sim.

[jbphet]

I'm unable to duplicate, and the commit that @aadish made seems like it would fix the issue. Back to @phet-steele to test on master and make sure that he can't duplicate it either.

[phet-steele]

Can't repro after that second commit.