Add short bar line; fix tick bar line

Add short bar lines (#6263 (closed))

\bar "," creates a short bar line and musicxml2ly knows how to use it.

The height of a short bar line is half the height of the staff, rounded up to an integer number of staff spaces. It is usually centered vertically, but on short staves, it is shifted down to distinguish it from a normal bar line.

Limit bar line extent to 2 staff-spaces by default (#6266 (closed))

The coping mechanism for choosing the height of bar lines on vertically short staves yields 2-space-high bar lines on a 1-line staff. Applying it only to staves shorter than 1 space created the strange situation that increasing the number of staff lines to 2 reduced the height of the bar lines, harming readability.

This change extends bar lines if they are less than 2 staff-spaces.

Testing exposed the separate issue that tick bar lines positioned themselves relative to the top of normal bar lines rather than the top line of the staff. (The outcome is often the same.) This change also fixes that.

Fix tick bar line thickness (#6264 (closed))

Use BarLine.hair-thickness to set the width of a tick bar line like other bar lines.

Other

• Add thickness to breathing-sign-interface. It was already used.
• Don't import Lily::ly_context_find. There is no need to call this via a Scheme import because it is directly available in C++ as find_context_above ().
• Fix typo in regression test description