Issue 3247: Prohibit non-postevents returned by music functions to be used as post...

Issue 3247: Prohibit non-postevents returned by music functions to be used as post-events

For example,

{ c-\mark \default }

produced an invalid music expression more or less indistinguishable from

{ \mark \default c }

Worse is that things like

{ c-\tweak color #red { g } }

also got accepted.  While \tweak (and other music functions) may
indeed return either post-events or straight music expressions, there
should be checks in place that the usage corresponds to its type.