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.

(cherry picked from commit a4e77e57f6)

app/file-data/file-data.h

@@ -0,0 +1,26 @@
+/* GIMP - The GNU Image Manipulation Program
+ * Copyright (C) 1995 Spencer Kimball and Peter Mattis
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+#ifndef __FILE_DATA_H__
+#define __FILE_DATA_H__
+
+
+void   file_data_init (Gimp *gimp);
+void   file_data_exit (Gimp *gimp);
+
+
+#endif /* __FILE_DATA_H__ */