Disallow empty 1.7 attribute values

Only in the parser, an empty template could still be used. Will need to
validate attributes somewhere else.

[SVN r86685]
Save values when parsing attribute value.

Needed in case there's any nested uses of values.

[SVN r86686]
Validate and process link values.

[SVN r86687]
Allow template calls in anchors, roles and includes.

Actually a subtle change, as includes and anchors could previously
contain spaces. Possibly should still alow that, although I expect it's
never used.

[SVN r86688]
Update 1.7 note about attribute templates.

[SVN r86689]
Inline element id version checks.

[SVN r86690]
Use quickbook 1.7's attribute value for element ids.

Allows use of templates, and allows more characters, ids now end with
whitespace - like ids in links.

[SVN r86691]
Check that attribute values aren't empty.

Maybe I should also make the parser more lax.

[SVN r86692]