Let \tweak on event chords manipulate its elements

Let \tweak on event chords manipulate its elements

This makes \tweak on event chords actually get heeded and creates
reasonably predictable behavior by tweaking the rhythmic elements (but
not chord-wide articulations).  It also passes tweaks transparently
through music wrapper expressions (like transpositions, context
changes and other).

To achieve this, it creates and uses a get-tweakable-music function
that will deliver a list of primary music expressions tweaks should be
applied to.

This function is also used in the definition of \once to decide when a
warning for misapplication of \once might be called for.


Also contains commits:

Refactor \once

Separates the warning action of \once and its effect on overrides in
order to reduce the number of surprises.


Let \single use \tweak internally

This is less efficient than directly manipulating the @samp{tweaks}
property but closer follows the semantics of @code{\tweak},
facilitating extensions to it.

[thomasmorley651, 2018-04-12 21:04:28]

LGTM