Proposal for removing area ids from block id

This pull request is just a proposal to try to resolve problems due to
moving blocks: when a block is moved its id still contains the previous
area id. What about removing the area ids from block-related html
nodes?

I don't know the possible side-effects, but from initial tests
everything seems to work fine.
One of the affected bugs is http://www.concrete5.org/developers/bugs/5-6-0-2/unexpected-error-while-moving-a-block-from-main-to-layout-area/

This pull request supersedes https://github.com/concrete5/concrete5/pull/712