Establish metatag convention.

Instead of trying to jam everything into type:x/y tags, I moved
type:text/x-brimlock-cal to app:brimlock/cal since you generally want an
item to have just one type:x/y to work with. That made me finally
realize that a metatag concept was needed where any tag beginning with
non-whitespace followed by a colon could be treated as a special tag not
normally used by a person but by code.
Fixed a couple Unicode related issues.
Added retag/rename-tag functionality.
Made a user-defined order option for tag lists.