Drop GTK2 support in favor of GTK3

[Jan-Marek Glogowski]

Am 14.08.19 um 12:03 schrieb julien2412:
> So what about putting 
> "The GTK+2 VCL backend is deprecated and will be removed in the next
> version."
> in https://wiki.documentfoundation.org/ReleaseNotes/6.3 ?
> So perhaps less people will complain.

People will complain anyway. They complained when we removed TDE, when
we removed KDE4 (much less) and I just had a bug report from someone
running gtk2 on LXDE, if I remember correctly.

It's easy to complain, if you don't have to do the support.

I imagine they will complain end of 2020, when qt6 will be there…
eventually. And when we drop X11 support for Wayland, but that's
probably past my lifetime.

Jan-Marek