kcms/kfontinst: somewhat modernized layout
Reason for the change
This is another attempted step at addressing kfontinst using way too much horizontal space and leaving too little room for the preview (which I reported as bug 493174). Compared to !4702, this gives back almost just as much space to the preview (after 4702, after this MR).
Instead of the filter + 3 columns, this reduces the UI to two columns, one for the group list and one for the filter, font list, and preview. This interface style is also used by font-manager (screenshot).
This also changes the separator between "standard" groups and custom groups to be much more like the separators elsewhere, instead of using the text color.
Additionally, this also changes item sizes to be consistent with other treeview pages and to be more touchscreen friendly, by no longer restraining groups and font families to KIconLoader::Small (16px) + 4px = 20px tall. Removing the size hint and letting the tree views do its thing leaves it at 32px, consistent with KDE Connect, Device Actions, Device Automounter, among others.
Moving the filter to solely on top of the font list should also fix bug 383129, where a user complained it's easy to forget the filter is active (since the filter currently sits on top of all three and so doesn't feel connected to the font list).
Test plan
I made sure adding groups still worked and that the separator no longer uses the text color.
I also reset the splitter sizes (rm ~/.config/kfontinstuirc) and made sure the default sizes are still reasonable.
Screenshots or screen recordings
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