contrib: qt: use more system libs

Some vendored libraries are quite up tp date in Qt (libjpeg), some not and print deprecation warnings now, e.g. harfbuzz 3.4.0 vs 1.7.4. But it should still work.

In any case, using the system variant is better than the vendored version. But some cannot be prevented (harfbuzz, see commit msg).

Followup !2666 (merged)