build/linux: Add super-privileged 'personal-files' interface to GIMP snap

See: #15547

This needs approval from Cannonical and allows migrating app data from
older versions as well writting current app data outside the sandbox.

build/linux/snap/snapcraft.yaml

@@ -41,6 +41,7 @@ apps:
       - intel-npu
       - npu-libs
       - gimp-plugins
+      - gimp-config
     extensions: [gnome]
 
 layout:
@@ -68,6 +69,10 @@ plugs:
     interface: content
     content: gimp-plugins-3
     target: $SNAP/gimp-plugins
+  gimp-config:
+    interface: personal-files
+    write:
+      - $HOME/.config/GIMP
 
 parts:
   buildenv:

libgimpbase/gimpenv.c

@@ -361,6 +361,23 @@ gimp_directory (void)
                                           NULL);
         }
 
+      const gchar *snap_path = g_getenv ("SNAP");
+      if (snap_path && g_file_test (snap_path, G_FILE_TEST_IS_DIR))
+        {
+          const gchar *snap_real_home = g_getenv ("SNAP_REAL_HOME");
+
+          if (snap_real_home == NULL)
+            gimp_dir =  g_build_filename (g_get_home_dir (),
+                                          ".config",
+                                          GIMPDIR, GIMP_USER_VERSION,
+                                          NULL);
+          else
+            gimp_dir =  g_build_filename (snap_real_home,
+                                          ".config",
+                                          GIMPDIR, GIMP_USER_VERSION,
+                                          NULL);
+        }
+
       if (gimp_dir == NULL)
         {
           /* g_get_user_config_dir () always returns a path as a non-null