app, plug-ins: start consolidating brush and pattern loading/saving code
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.
Showing with 458 additions and 378 deletions