i18n: drop po-properties/ directory

This is a major i18n refactoring: the property i18n has been merged into
the main .pot file and the intltool crap has been dropped. The build
system has been modied accordingly and gettext scripts have been update
to current (gettext-0.19.3). Further changes have been applied to avoid
deprecation warnings.
i18n: update italian translations

This, together with the previous commit, basically closes issue #101:
http://dev.entidi.com/p/adg/issues/101/
i18n: drop relic convention in string quotation

Once upon the time there was `this' quote style. Dropped in favor of a
'single' quote convention. Also, avoid dropping numbers, as they are
already different enough from the surrounding text.
i18n: update italian translations