disable transitions on vertical panel layout

mitigates #539
(there's still a last little stripe that takes a second to go away)
don't allow new notebook click to propagate

fixed #545
Merge branch 'develop' of https://github.com/att/rcloud into develop
show and then hide grab affordance for readonly

to make consistent with other readonly controls
the mvc here is kind of wacky because cells never change readonlyness
but i don't want to mess with it

fixes #546