Add role="region" directly to the offcanvas

[patrickhlauke]

Fixes the (valid) complaint from vnu #2221 (comment)

[patrickhlauke]

Ah, actually, I forgot that that <div> in large viewport is just a regular container that is actually visible. so dialog won't be appropriate. let me tweak this...

[patrickhlauke]

the fact that the offcanvas script doesn't keep track of whether or not the container had a role to begin with, and after closing simply deletes the role attribute altogether...is probably not ideal. but that's a different problem to tackle (scripts that add and remove things like roles should really make sure they return gracefully to whatever the author had originally specified in markup)