community/firefox{,-esr}: split gtk2

It is only required for Flash support: https://bugzilla.mozilla.org/show_bug.cgi?id=1377445. Most users will not require it, so we can make it optional. See for example https://packages.debian.org/buster/firefox-esr: "sug: libgtk2.0-0".

I haven't tested this, but I think if it passes CI and has the correct dependencies it should work.