min version of gtk3

[Caolán McNamara]

On Fri, 2017-12-01 at 13:31 +0000, Caolán McNamara wrote:
> On Fri, 2017-12-01 at 14:25 +0100, Jan-Marek Glogowski wrote:
> > You want to drop Gtk+ < 3.20.0 support in or after 6.0?
> 
> For me I was thinking for > 6.0, scarabeus (for suse) was wondering
> additionally about 6.0

I've bumped the min requirement to build and run to gtk3 3.18 and
propose to raise that to 3.20 for master if there's no objection to
that https://gerrit.libreoffice.org/#/c/45735/