Support OpenType font features through fontconfig

Fontconfig supports OpenType features, but it looks like Qt does not handle this.  As a concrete example, create a file called ~/.config/fontconfig/conf.d/00-features.conf with the following:

 
<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<fontconfig>
    <match target="font">
        <test name="family">
            <string>Source Code Pro</string>
        </test>
        <edit name="fontfeatures" mode="assign_replace">
            <string>zero</string>
        </edit>
    </match>
</fontconfig>

Explanation:

The font Source Code Pro has, by default, a dotted zero. But the font includes the "zero" OpenType feature which provides a slashed zero instead. The fontconfig rule above will enable the zero feature for Source Code Pro.

This works in Pango-based applications, but does not appear to work in Qt. Pango applications get the slashed zero, but Qt applications still get a dotted zero.

It'd be great if Qt could support OpenType features via fontconfig.