Depending on hardware and recorded volume level, turning up the speakers
to the maximum volume may not be enough and the user will want to amplify
the volume above 100%. Currently this requires opening the sound Settings
panel which gets cumbersome when required repeatedly.
To support this case better, allow raising the sound volume above 100%
directly from the system menu if the feature is enabled via the
`allow-volume-above-100-percent` key in `org.gnome.desktop.sound`.
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