gnu: Reduce the number of dependents on inkscape.

This reduces the number of dependents from close to 2000 packages to less than
100.  This is done by substituting gtk-doc for gtk-doc/stable in a few
packages: the latter depends on inkscape/pinned rather than inkscape.

* gnu/packages/display-managers.scm (lightdm)
* gnu/packages/freedesktop.scm (accountsservice)
(libqrtr-glib, libappindicator)
* gnu/packages/gnome.scm (gcr, upower, libzapojit, gnome-software)
* gnu/packages/gtk.scm (cairo-with-documentation, gtk, libdbusmenu)
* gnu/packages/ibus.scm (ibus-minimal)
* gnu/packages/mail.scm (gmime)
* gnu/packages/music.scm (libgpod)
* gnu/packages/package-management.scm (flatpak)
* gnu/packages/xfce.scm (xfce4-dev-tools)
[propagated-inputs]: Remove gtk-doc; add gtk-doc/stable.

gnu/packages/ibus.scm

@@ -216,7 +216,7 @@
            gnome-common
            gobject-introspection        ;for g-ir-compiler
            `(,gtk+ "bin")
-           gtk-doc
+           gtk-doc/stable
            perl
            pkg-config
            python-wrapper