left justification of mensurations and section titles

[craigsapp]

A mensuration or new section starts at a given barline, so techncially these symbols should be left-justified to the measure they start in rather than centered:

Better as:

[craigsapp]

Left-justification will fix this case at the start of:

http://ribbon.humdrum.org/?id=Jos2510

[craigsapp]

Note that it is possible to have section titles overlap (which would not be fixed by left alignment):

[curran]

Closed in #182