Use a '$' instead of a '.' to indicate a type provided in application
code.
The reason for the change is to have a consistent "extern" symbol that
isn't widely used elsewhere and isn't ambiguous in expressions.
Inline menus didn't work anyway--menus have to be referenced by ID
(though, curiously, you *can* put the <menu> within the <property> tag
and immediately reference it--but that's a hack, and not what
blueprint-compiler was doing).
I want all language elements to be defined in the same folder, rather
than having the "core" language split by parsing/AST/completions and the rest
split by language element.
