nixos/gnome: add glycin-thumbnailer and gst-thumbnailers

These are the new multimedia thumbnailers developed for GNOME.

gst-thumbnailers in particular is useful because it adds support for
video thumbnailing back to GNOME. Video thumbnailing was previously
provided by totem, but totem isn't shipped by default anymore as it was
replaced by showtime.

Adding them to `optionalPackages` as they are not core apps as per [1].

[1] 6be2940aa3/elements/core/meta-gnome-core-apps.bst

nixos/modules/services/desktop-managers/gnome.nix

@@ -439,6 +439,7 @@ in
           optionalPackages = [
             pkgs.adwaita-icon-theme
             nixos-background-info
+            pkgs.glycin-thumbnailer # Image thumbnailers
             pkgs.gnome-backgrounds
             pkgs.gnome-bluetooth
             pkgs.gnome-color-manager
@@ -447,6 +448,7 @@ in
             pkgs.gnome-user-docs
             pkgs.glib # for gsettings program
             pkgs.gnome-menus
+            pkgs.gst-thumbnailers # Audio and video thumbnailers
             pkgs.gtk3.out # for gtk-launch program
             pkgs.xdg-user-dirs # Update user dirs as described in https://freedesktop.org/wiki/Software/xdg-user-dirs/
             pkgs.xdg-user-dirs-gtk # Used to create the default bookmarks