Split gtk-update-icon-cache utility out of libgtk-3-bin into a separate

package called gtk-update-icon-cache to break a dependency cycle with
adwaita-icon-theme when building the package. (Closes: #747392)
As the gtk-update-icon-cache binary doesn't actually have any libgtk-3-0
dependency, there is no longer a reason to ship a gtk2 and gtk3 variant.
So from now on we will only ship one implementation built from src:gtk+3.0
and drop the Debian specific -3.0 suffix. To simplify the transition make
libgtk-3-bin depend on gtk-update-icon-cache. Once all theme packages have
been updated, this dependency can be dropped.