Make inherited stream event properties immutable.

Make inherited stream event properties immutable.

Previously, the initialization of stream events from the (mutable) part
of music event properties was done by just taking the mutable property list.
That had the effect that any changes to the inherited properties were
reflected in the originating music event, making a mockery of the
mutable/immutable distinction intended to provide copy-on-write
semantics.  As an example, the conversion of the articulations
property to events might or might not already have happened if the music
event got reported more than once, like it is done in \repeat unfold.

The change had the unexpected side effect of making quoted transposed
music fail since the transposition worked exclusively on the mutable
property list part.  This has been fixed by adding a make_transposable
member function to Stream_event that copies affected properties from
the immutable to the mutable part before one attempts transposition.

The source code has not yet been combed through for instances where
stream events have been manually cloned to avoid the bad side effects
of the original code.