raise min version of gtk -> 3.20

[Caolán McNamara <caolanm@xxxxxxxxxx>]

Back in Dec 2017 we considered raising the min required version of gtk
to 3.20, but ultimately I raised to just 3.18.

I'd like to now raise that to 3.20 for LibreOffice 7.1
https://gerrit.libreoffice.org/c/core/+/103901
mostly so we can remove a much unloved complicated hack to support two
variants of themeing.

Any concerns, let me know.

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