Fix crashes in woodwinds with key names

The fix for tin-whistle is by Thorsten, see https://lists.gnu.org/archive/html/bug-lilypond/2020-05/msg00030.html

For saxophone, there were two issues: Left hand keys could not be displayed because it incorrectly used make-right-hand-key-addresses (found by Valentin). Moreover, low-a could not be used because it defines (text? . #f). Fix this by using the custom text stencil.

Closes #5990 (closed), supersedes !68 (closed).