measureStartNow context property and related clean-up
Timing_translator: set measureStartNow context property
Move new-measure detection from Default_bar_line_engraver into Timing_translator.
Also clean up Multi_measure_rest_engraver, which was reading the whichBar property as an indirect way to detect a new measure.
It is possible that other readers of measurePosition or callers of measure_position () could use measureStartNow instead, but I have not considered any of them thoroughly.
Clean up measure_length() and measure_position().
measure_position () could possibly be used in more places instead of raw access to the measurePosition property, but I held back because I didn't want to investigate every call to figure out whether bypassing the normalization that measure_position () does is important.
Paper_column_engraver: remove useless last_moment_
It was never updated.