gnome abstraction: allow reading per-user themes from $XDG_DATA_HOME

Bug-Debian: https://bugs.debian.org/930031

As per https://developer.gnome.org/gtk3/stable/ch32s03.html,
since GTK+ 3.6, $XDG_DATA_HOME/themes is preferred over $HOME/.themes.
We already support the latter, let's also support the former.

profiles/apparmor.d/abstractions/gnome

@@ -51,6 +51,8 @@
   owner @{HOME}/.gtk-bookmarks          r,
   owner @{HOME}/.themes/                r,
   owner @{HOME}/.themes/**              r,
+  owner @{user_share_dirs}/themes/      r,
+  owner @{user_share_dirs}/themes/**    r,
 
   # for gtk file dialog
   owner @{HOME}/.config/gtk-2.0/                    w,