Issue 4687/1: Let \etc in partial markup commands stand for any arguments

Issue 4687/1: Let \etc in partial markup commands stand for any arguments

Previously, \etc could only stand for a final markup argument.  Now an
arbitrary kind and number of arguments from the last markup command in
a markup command chain may be omitted.

Example:

quarter-markup = \markup \note-by-number #2 \etc

\markup { 3/8: \quarter #1 #UP }