Link Model is now respected for Tag Set Props

That is, if you define a Tag Set Property with a specific Link Model,
when you go to reify one of those Tags, it will use that Model.

Added PType.matches, to provide a consistent way to compare VT values.
(Needed for NameType, so that it compares case-insensitively.)

Added Property.confirmType, as a way to work around type-math
weaknesses.  Used that to make Space.propsOfType more type-aware.
Started using Manifest for Tag Sets

Not using MarcoPolo yet, so no prompting, but plain Manifest makes
things look better.
Tags now work as designed

Implemented full MarcoPolo support for tags.

Added the /getTags AJAX entry point to support MarcoPolo.
Tag input now prompts for Things as well

Specifically, if it can't find any Tags that fit what you are typing, it
widens the search to include the names of Things as well.