Issue 2705: Parse music inside of output definitions in "notes" mode.

Issue 2705: Parse music inside of output definitions in "notes" mode.

This makes \midi { \tempo 4. = 120 } work properly (without parsing
4. as a REAL token).

The next token after music will in rare circumstances be scanned still
in the lexer's "notes" mode.  This could be problematic because of the
currently different word syntax of "notes" and "INITIAL" mode
(cf. issue 2702).