"Mixing liquids" model: make rectangles taller

[#55054750]

... to prevent atoms from falling out.
MML->JSON parser: text box color definition can use object reference
Fix lines arrowheads

[#45184223]
Create model and interactive presenting custom arrowheads

[#45184223]
Rename shifted-arrowheads.json to arrowheads.json

[#45184223]

As this interactive doesn't present shifted arrowheads anymore.
Remove small, unnecessary lines from "Volume-Pressure interactive"

[#55156950]
pulldown-controller: revert change, fixes Android issue

[#55315864]

I'm punting on fixing this properly for the 0.5.9 release and
instead I'm restoring src/lab/common/controllers/pulldown-controller.js
to the state it was before the recent changes.This reverts changes Richard made to pulldown-controller.js in
the following commits: a9999773, b8af59d1, 74bdd963.

This commit will be reverted in master after the 0.5.9 release.

see thread: https://groups.google.com/forum/#!topic/lab-models/MEiebH-rUEY