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.
`blueprint-compiler port` interactively ports a project to blueprint.
It will create the subproject wrap file, add it to .gitignore, decompile
your GtkBuilder XML files, emit code to copy and paste into your
meson.build file, and update POTFILES.in.
It can't quite handle all of the features the forward compiler can, so
it will skip those files.
