Add definitions for the gtk-update-icon-cache(1) command to the
script header template, and a makefile fragment that creates the
data file for the "icon_themes" package task if ${ICON_THEMES} is
"yes" in the package Makefile.
Also bump pkgtasks dependency to version 1.12 for the "icon_themes"
package task and compatibility with pkgsrc/mk/pkginstall.
