Issue 3603: Make \tweak ... Grob equivalent to \override rather than \once \override

Issue 3603: Make \tweak ... Grob equivalent to \override rather than \once \override

This simplifies several uses of tweak for both tweak and override.
When desired, one can use \once \tweak in order to get either tweak or
override-once semantics: in the case of the tweak semantics, \once
will just ignore the contained expression.