make menuitemradio subclass menuitem, require aria-checked

[carmacleod]

Closes #1347.

This PR:

• changes the superclass of menuitemradio to be menuitem (instead of menuitemcheckbox)
• makes aria-checked a required attribute of menuitemradio (was previously inherited from menuitemcheckbox)

Also, because menuitem superclass is now obviously a Related concept:

• changed menuitem to menuitemcheckbox under the Related concepts for menuitemradio
• changed menuitem to menuitemradio under the Related concepts for menuitemcheckbox

@WilcoFiers - would be great if you have a chance to review this. :)

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Preview | Diff

[JAWS-test]

The class diagram should be updated too because it is linked at 5.3 Categorization of Roles

[jnurthen]

@JAWS-test The class diagram is no longer maintained. If you follow the link it states this.

[JAWS-test]

I thought there will be a new one: #1222

[carmacleod]

@jnurthen I think @pkra's Tree diagram from #1222 is quite nice, and we should link to it for ARIA 1.3.
(Thanks for the pointer, @JAWS-test).

At a minimum, we could update it and put it at a 1.3 variant of: https://www.w3.org/WAI/ARIA/1.2/class-diagram/