o * lily/spacing-engraver.cc (stop_translation_timestep): directly

o	* lily/spacing-engraver.cc (stop_translation_timestep): directly
	copy proportionalNotationDuration into currentMusicalColumn if
	set.

	* lily/spacing-spanner.cc (musical_column_spacing): if
	uniform-stretching set, fixed space is 0.0

	* input/regression/spacing-multi-tuplet.ly: show uniform-stretching property.

	* lily/spacing-spanner.cc (effective_shortest_duration): new function.

	* lily/include/spacing-spanner.hh (class Spacing_spanner): new file.
	(struct Spacing_options): new struct.

	* lily/spacing-basic.cc: new file.

	* lily/spacing-spanner.cc (note_spacing): remove outdated code:
	delta_t doesn't have to be smaller than shortest_playing_len