When a custom library is created, it's definition
file gets written to the user directory. If the
project is exported to another user, or if the
current user deletes or changes their user
directory, the project contains a dangling library
reference.

There needs to be a parameter (and corresponding
UI element in the LibraryCustomizer) to specify
that the library definition should be written to
the project directory say,
nbproject/config/org-netbeans-api-project-libraries/Libraries
This will allow projects to be self contained in
terms of their library references.

I realize this will affect how the LibraryManager
works, since it currently is not sensitive to any
particular project context. It could however,
either show a node for the Main project, or a node
for each open project in the IDE.