Allow a tighter gap between system header time signature and start repeat barline

At the start of a system, a time signature followed by a start-repeat barline always keeps a minimum gap of at least 1.5 sp, and no style setting can reduce it.

The cause is in HorizontalSpacing::minHorizontalDistance(), which applies to system headers only:

1. The "Time signature to barline" (0.5 sp) is maxed with the "Time signature to first note" (2.0 sp), and therefore overridden (with the default values).
2. For a start repeat, endBarWidth + endBarDistance (0.92 sp by default) is subtracted, to align the repeat's thin line to where the header ends.
3. A hardcoded minimum of 1.5 * spatium (absoluteMinHeaderDist) is then applied to the clearance measured to the barline's left edge.

With the default values, this is 2.0 − 0.92 = 1.08 < 1.5, so the minimum 1.5 is applied and both gap style settings appear useless. "Time signature to barline" only starts to have any visible effect once it surpasses 1.5 + 0.92 = 2.42 sp. The alignment from step 2 is also defeated by this minimum: the thin line ends up 2.42 sp from the time signature rather than the requested 2.0 sp.

https://github.com/user-attachments/assets/f7868d0c-9401-4353-a43c-f54963de1b82

The same affects clef/key signature headers via the systemHeaderDistance style.

This minimum is a too wide by default, we should allow the user to tighten this gap.