Fix #306524: Control playback speed with the keyboard

[Spire42]

Resolves: #306524

Implemented three new commands that increase, decrease, and reset the playback speed to 100%. The speed increment defaults to 5% but is configurable via an advanced preference. The new commands are not assigned default keyboard shortcuts but are configurable by the user.

Also renamed “relative tempo” to “playback speed” in the relevant code, since that more accurately describes what it actually is. This distinction will become more important later if (or when) MuseScore gains true relative-tempo control at the score level (as opposed to the playback/transport level).

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