fix #305215 : Crash when importing a file with an incomplete tuplet

[AntonioBL]

Resolves: https://musescore.org/en/node/305215

This PR inverts the order of fillGapsInFirstVoices() and resetTuplets(), as suggested by Leon Vinken in the issue tracker.
It also introduces fraction calculation in pass 1 of the import to determine measure duration, and it tries take into account small residuals deriving from integer rounding errors.

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