remove <menu> and <menuitem>

[siusin]

Clean up the at-risk features in #893 :

• remove the<menu>element;
• remove the <menuitem> element;
• remove Context menu;
• remove the contextmenu event;
• keep menubar which is defined in wai-aria-1.1;

[chaals]

Please add the changes to sections/changes.include

Otherwise, Looks good to me, although it's hard to check that everything came out :S

[siusin]

Added a "Changes since the 2017-08-08 Candidate Recommendation" section.

[chaals]

Thank you - and for fixing stuff that I should have done in my own PRs...