(fix/Qt6) Skins: hide reserved space for invisible table header sort indicators

[ronso0]

before:

after:

Working around a Qt 6 regression, probably this one again https://bugreports.qt.io/browse/QTBUG-27038
(I don't care about reporting this upstream with a minimal reproducible example, usually takes ages until being fixed)

Issue:

QHeaderView will reserve space in the header section for sort indicators, even if they're invisble as soon as we set either an image

WTrackTableViewHeader::up-arrow {
  image: url(skins:LateNight/classic/buttons/btn__lib_sort_up.svg);
}

or the width

WTrackTableViewHeader::up-arrow,
WTrackTableViewHeader::down-arrow {
  width: 0.8em;
}

Fix:

Set a negative padding-right to push the space out of the visible region.
The indicator will be inside nonetheless 🤷‍♂️

Please test on Windows and macOS.

[JoergAtGithub]

LGTM! Thank you!