Allow style attributes to be pasted from other VE instances

Use the new data-ve-attributes to preserve styles when copying
between VE instances. Previously we always threw away styles as
we couldn't trust them not to be modified.

As the browser can also add in styles where there were none
(unlike RDFa attributes) we have to remove all style attributes
before restoring data from data-ve-attributes.

Change-Id: I71eb211c29edc2f71c614a541c74e2d89590b72e
Merge "Allow style attributes to be pasted from other VE instances"