gnome-extra/gnome-calendar-3.26: dep fixes, USE=gtk-doc support

gnome-extra/gnome-calendar-3.26: dep fixes, USE=gtk-doc support

Hook up IUSE=gtk-doc for developer documentation.
Add explicit glib-utils build dep (lost in bump).
Remove by now redundant meson minimum dep, eclass ensures newer.
Add missing explicit libxml2:2 build dep as strip-blanks is used
in gresource files.
Icon cache update is handled in xdg.eclass now, so we don't need
the explicit gnome2_icon_cache_update calls anymore.
Restrict tests if USE=test is explicitly disabled.

Package-Manager: Portage-2.3.52, Repoman-2.3.12
Signed-off-by: Mart Raudsepp <leio@gentoo.org>