Allow recorded sample tracks to be saved as distinct files

### Enhancement Summary
PR #5990 will give us basic sample recording support. While we now can record new samples on the fly, there is currently no way to save these samples as distinct files (at the moment this can only be achieved by putting the track on solo and render the song, but this would be rather an ugly hack than a proper UI).
Working on this makes only sense after PR #5990 has been merged.

#### Justification
One might want to save a recorded track for post-processing in another application.

#### Mockup
A recorded sample track could have a `save as` option in its right-click context menu to save it to disk, maybe even in different formats (wav, mp3, ogg, ...; whatever the export dialog offers as well). This could also be used for normal sample tracks (though it is probably not necessary there).

First verify that PR #5990 has been merged and review the sample recording support it adds. Then trace the recorded sample track's right-click context menu and the existing export dialog; done means a recorded sample can be saved as a distinct file in a supported export format.

Written by the indexing model from the issue text.