SpinBox.increase() / decrease() are not callable in Qt 6.11

[trin94]

Since the migration to Qt 6.11, SpinBox.increase() and SpinBox.decrease()
are no longer callable as functions. We work around this by reproducing the
step-and-clamp behaviour in QML.

Upstream: QTBUG-145174

Affected areas

• qt/qml/views/table/MpvqcEditTimePopup.qml (production workaround)
• qt/qml/components/tst_MpvqcSpinBoxRow.qml (test-side workaround)

Drop the workarounds and call SpinBox.increase() / decrease() directly once
this is fixed upstream.

Environment

• mpvQC 0.9.0-beta3 (commit cf469d0)
• Qt 6.11