Fix #24723: Honor --score-transpose flag in CLI

[jongbinjung]

Resolves: #24723

While investigating #24723 three problems were identified and addressed in this PR:

• A temporary "filename" with .mscx extension that should be .mscz (but immaterial)

• BackendApi::parseTransposeOptions was not setting options.interval and options.key before returning

• Since 4.0, Score::transpose only applies to a selected range. In the GUI, this is accommodated by selecting all when the TransposeDialog is opened without a selection. Because the CLI will never have a selection, treat it like opening the TransposeDialog with no selection and replicate the behavior of selecting all.

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[cbjeukendrup]

Tested on Windows, looks good!