Introduce Icon Theme cache handling framework
Icon Theme cache files are used by GTK+ and maintained with the
gtk-update-icon-cache tool. Each Icon Theme package duplicates
its own maintainance scripts: only the specified icon theme directory
differs. With this framework, if packages have ICON_THEMES=yes,
associated icon themes will be detected and their cache files will
be maintained automatically.
Change cache handling behaviour as follows:
* Icon theme caches will be updated if either gtk2+ or gtk3+
gtk-update-icon-cache tool is available.
* With installation of gtk2+ package, not only hicolor icon theme but
also any other icon theme cache files will be updated.
* Prevent removal of icon caches at deinstall, gtk3+ may be installed and
using them.
* Ditto with gtk3+, gtk2+ may not be installed now, so caches must be
maintained by gtk3+.
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