#1677 Mixers don't visually scale

[Lorenzo]

It seems both the MIDI and Audio mixers don't scale visually.

By 'scale' I mean that if one resizes the window the widgets all remain at the same size and padded instead of scaling which would be particularly useful on higher resolution displays.

For example in the screenshots attached, MIDI and Audio Mixers maximised on my laptop which has a resolution of 1920x1080 an makes the mixers barely usable.

My system:
- Manjaro with KDE Plasma 5.27.10 and QT 5.15.12
- The display settings are set to 100% scale

Overall Rosegarden seems to play well with my system settings set in KDE Plasma in terms of theme, fonts etc. so this seems specific to the mixers