[filedialog] Port to KFileFilter-based API

Nicolas Fella requested to merge work/nico/filefilter into master

This matches what we get from the front-end more closely, thus simplifying the code and making it more correct

The mobile part isn't properly making use of it yet

CCBUG: 450612

Needs frameworks/kio!1466 (merged)

Edited by Nicolas Fella

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