Bug 499885

Created attachment 178186 [details]
Fonts settings

SUMMARY
Font features configured for the Inter font family are not being properly applied and persisted in Plasma Desktop. The issue specifically affects OpenType font features.

STEPS TO REPRODUCE
1. Go to "System Settings" -> "Text & Fonts" -> "Fonts"
2. Choose a font that supports Font Features, e.g. Inter
3. Add a feature from https://rsms.me/inter/#features, e.g `ss02` into "Comma separated font features (e. g. liga, calt):"
3.  Click the OK button.
4. Click the OK button (or Apply and OK).

OBSERVED RESULT
The font feature is not applied. Upon logout and login the entered `ss02` disappears completely from the Fonts setting.

EXPECTED RESULT
To be able to use font features to make certain fonts more eligible.

SOFTWARE/OS VERSIONS
Operating System: EndeavourOS 
KDE Plasma Version: 6.2.5
KDE Frameworks Version: 6.10.0
Qt Version: 6.8.2
Kernel Version: 6.13.2-arch1-1 (64-bit)
Graphics Platform: Wayland

ADDITIONAL INFORMATION
I've tried using custom fontconfig at `~/.config/fontconfig/conf.d/20-inter-features.conf` to apply a font feature, but it only worked for certain applications like Firefox, and not for System Settings, for example.

```xml
<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<fontconfig>
	<match target="font">
		<test qual="any" name="family"><string>Inter</string></test>
		<!-- These are set with "prepend" so that applications which configure features don't get overridden -->
		<edit name="fontfeatures" mode="prepend">
			<string>cv05</string><!-- Lower-case L with tail -->
		</edit>
	</match>
</fontconfig>
```