\tweak no longer works with \tuplet

[Werner LEMBERG]

Folks,


have a look at 

  https://sourceforge.net/p/testlilyissues/issues/5555/

This is a serious regression.

I've noticed the problem since I'm going to rewrite the Stockhausen
Klavierstück II to fix various issues accumulated over time (and to
make the input look less intimidating).

In the bug report I've also provided a regression test; it seems we
missed this issue completely because `\tweak' + `\tuplet' is not used
in any of the regression files.


    Werner