Remove ability to lock to angles and add ability to lock to 'natural' orientation.

Solves the following bug: https://www.w3.org/Bugs/Public/show_bug.cgi?id=25329

This push has two commits:
"Remove the ability to lock to angles." that removes the support for locking to 0, 90, 180 and 270.
"Add 'natural' orientation lock type." that adds support for locking to 'natural' which is equivalent to locking to angle 0.

Please ignore the following commit:
"Define what the term 'screen' means in the context of the specification."
Is that closing `</a>` there a stray tag?