dolphin: add qtimageformats

This allows Dolphin to render thumbnails/previews for more image
formats, like webp.

pkgs/applications/kde/dolphin.nix

···
   kcompletion, kconfig, kcoreaddons, kdbusaddons,
   kfilemetadata, ki18n, kiconthemes, kinit, kio, knewstuff, knotifications,
   kparts, ktexteditor, kwindowsystem, phonon, solid,
-  kuserfeedback, wayland, qtwayland, qtx11extras
+  kuserfeedback, wayland, qtwayland, qtx11extras, qtimageformats
 }:
 
 mkDerivation {
···
     kuserfeedback
     wayland qtwayland
     qtx11extras
+    qtimageformats
   ];
   outputs = [ "out" "dev" ];
 }