Issue 5123 Improve elbowed-hairpin

Issue 5123 Improve elbowed-hairpin

Let the lines be printed by the new make-connected-line-procedure,
using ly:line-interface::line. The new stencil now reacts on
overrides for style and dash-period/fraction.
Not closing Hairpins created by elbowed-hairpin are possible now.
Single disadvantage: The point-list needs to have (0 . 0) first,
if a closing Hairpin is wished.
The previous used make-connected-path did that automatically and
thus was not flexible enough.

Give the final stencil proper extents.

Cleanup, more descriptive naming, extent docstring.