Add configurable playback speed step

[pederbe]

Summary

• add a configurable playback speed step to the General preferences, defaulting to the existing 0.10x increment
• apply the configured value to the existing inc_speed and dec_speed actions, including keyboard and mouse-wheel controls
• keep the existing action IDs and fixed 4% and 1% speed actions for compatibility

Testing

• built the release target on Windows with Qt 5.15.2 and MinGW 8.1
• verified the command-line help smoke test
• manually verified changing the setting and adjusting speed during playback with the keyboard and mouse wheel
• manually verified remembered playback speed when opening other files
• ran git diff --check

Closes #1387