Complement concept of primary output with priorities order

Port and adapt to [changes in libkscreen](libkscreen!91). Adjusted config serialization format, KCM UI/UX, and generator's logic.

New KCM UI: frameworks/kirigami!843 (comment 571485)

Reordering list could use some drag & drop, but the original model isn't usable for that as is, and sort-filter-proxy seems to not be emitting "rows moved" signal as it should, so I didn't bother implementing custom crafted model on top of existing one. For ≤2 outputs case it might make sense to present old UI with a "primary" checkbox, except that checkbox was never a good fit for this job (it can't be unchecked, but if you manage to — here be dragons).

See also: libkscreen!91

See also: plasma-workspace#52