channelwidget: Make volume strings translatable and add missing space

The volume strings (e.g. ‘86% (-3.81dB)’) shown to the right of each
volume slider is now translatable/localisable. (The similar string
‘100% (0dB)’, centred *below* each slider, was already translatable.)
Also, the English strings now include a space between the decibel value
and the unit, e.g. ‘-3.81 dB’ instead of ‘-3.81dB’.

Fixes: #52