Give KDE Desktop Settings window a few seconds to stabilize

Rawhide had a failure today where the dropdowns moved between
us matching and clicking, so we clicked the wrong one:
https://openqa.fedoraproject.org/tests/1005662

Signed-off-by: Adam Williamson <awilliam@redhat.com>

lib/utils.pm

@@ -1398,6 +1398,8 @@ sub solidify_wallpaper {
         send_key 'd';
         send_key 's';
         release_key 'alt';
+        # give the window a few seconds to stabilize
+        wait_still_screen 3;
         # Select type of background
         assert_and_click "deskset_select_type";
         wait_still_screen 2;