Show TupletBrackets also on single notes

Show TupletBrackets also on single notes

While tuplet brackets should not be printed for tuplets consisting
of only one note (e.g. a note with tremolo!), the tuplet number still
needs to be printed and can not be ignored.

Since the tuplet number also uses the dimensions calculated by the tuplet
bracket, the tuplet bracket at least needs to calculate it's theoretical
size, too, even though it will not be printed for single-note tuplets. The
check to kill zero-length tuplet brackets has thus been moved from
calc_positions to print in Tuplet_bracket.

Signed-off-by: Reinhold Kainhofer <reinhold@kainhofer.com>