Issue 3100: Create \tuplet function and use it everywhere.

Issue 3100: Create \tuplet function and use it everywhere.

This is the big version of issue 3095: it creates convertrules for
converting all the \times statements into \tuplet.  It also folds
immediately preceding \set tupletSpannerDuration into the call.
Consists of the following patches (latest to first):

Run scripts/auxiliar/update-with-convert-ly.sh


define-music-display-methods.scm: use \tuplet rather than \times for
TimeScaledMusic


convertrules.py: substitute \times with \tuplet where appropriate


Create a \tuplet function to complement \times and tupletSpannerDuration

Here is its DOC string:

Scale the given @var{music} to tuplets.  @var{ratio} is a
fraction that specifies how many notes are played in place of the
nominal value: it will be @samp{3/2} for triplets, namely three notes
being played in place of two.  If the optional @var{duration} is
specified, it is used for initializing @code{tupletSpannerDuration}.
For example,
@example
\tuplet 3/2 4 { c8 c c c c c }
@end example
will result in two groups of three tuplets, each group lasting for a
quarter note.  The value of @code{tupletSpannerDuration} is retained
as default if it is set in this manner.  Clearing it can be achieved
by specifying @samp{1*0} as @var{duration}.