Fix some issues, warnings + improvements.

sid: xfce4-settings: 4.18.6-2

Hi, Dear Maintainer. I made some corrections for xfce4-settings:

• updated d/upstream/metadata file (actualization, added new Fields)

• remove empty variable ${shlibs:Depends} from d/control for package xfce4-helpers

dpkg-gencontrol: warning: Depends field of package xfce4-helpers: substitution variable ${shlibs:Depends} used, but is not defined

• fix lintian warnings:

superfluous-file-pattern (remove dialogs/appearance-settings/images.h from d/copyright because it doesn't exist)

desktop-command-not-in-package (override this, added lintian-overrides for many these warnings). I think: Dependency on exo-utils ensures exo-open is available, so warning is non-critical.