Implement basic font-feature-settings widget.

This adds the ability to edit CSS Font Feature Properties, which should complete the opentype features.

This also enables turning off font-kerning, before I forget.

To do:

• Make feature add dropdown provide some kind of completer, ideally with all possible tags.
• Also check GPOS instead of only GSUB like right now.
• Use the samples somehow.
• Double check with ASAN.

Test Plan

• Junicode VF has by far the most font features from what I've seen.
• Add new opentype features entry.
• Use it to add features.
• Type in an unavailable feature like "proportional kana" in a non-cjk font. (This should work, it allows us to not worry about features on fallback fonts.)

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.
• Does the patch add a user-visible feature? If yes, is there a documentation MR ready for it at Krita Documentation Repository?

Reminder: the reviewer is responsible for merging the patch, this is to ensure at the least two people can build the patch. In case a patch breaks the build, both the author and the reviewer should be contacted to fix the build. If this is not possible, the commits shall be reverted, and a notification with the reasoning and any relevant logs shall be sent to the mailing list, kimageshop@kde.org.