We currently have brush and pattern I/O code in both the core and
plug-ins. This commit starts removing plug-in code in favor of having
one copy of the code in the core, much like XCF loading and saving is
implemented.

Add app/file-data/ module with file procedure registering code, for
now just with an implementation of file-gbr-load.

Remove the file-gbr-load code from the file-gbr plug-in.