mainwindow: change "[Set as] fallback" to "default"

This came up on #pipewire, and as turned out many people have been making same mistake. Let's fix this source of confusion once and for all.

The "Set as fallback" title is a frequent source of confusion, many people look at it and do not realize that its actual functional is "setting as default". Presumably, when this title first appeared it was presumed that apps should chose a device themselves, and then if they don't there's a "fallback". But in reality most apps do not chose any device explicitly, and it's expected behavior to make use of this "fallback" device. So it is more correct to call a "default" device.