gtk icon cache

[Patrick Welche <prlw1%cam.ac.uk@localhost>]

Even before the gtk updates, our icon themes make use of
gtk-update-icon-cache in their INSTALL/DEINSTALL templates,
but do not have a dependency on gtk, so there is no guarantee
that gtk-update-icon-cache is present when the icon theme is
installed.

Do we want to fix (guarantee) this and introduce a dependency?

If yes, then:

GTK3:
- adwaita-icon-theme
GTK2:
- elementary-xfce-icon-theme
- gnome-icon-theme
- hicolor-icon-theme

?

Cheers,

Patrick