Proposed context property renaming

[Dan Eble]

    Current Name                    Proposed New Name
    ------------------------------  ---------------------------
    measureLength                   measureLength (no change)
    minimumPageTurnLength           pageTurnMinimumRestLength
    minimumRepeatLengthForPageTurn  pageTurnMinimumRepeatLength
    proportionalNotationDuration    (not sure yet)
    tupletSpannerDuration           tupletBracketMusicalLength
    voltaSpannerDuration            voltaBracketMusicalLength

The motivation for moving away from "duration" is that there is an 
internal type called duration and that is not what these use.

There is inconsistency between "length" and "musical length" because it 
sounds unnecessary to specify that "length" is used in the musical sense 
for measure, rest, or repeat, but it seems helpful to specify it for a 
horizontal bracket.

Do you agree?
--
Dan