Let users select Jack outputs from combo box

[michaelgregorius]

Enhancement Summary

Users currently need to use an external connection management tool to connect LMMS' outputs to a Jack output (or rather "Jack input" in Jack parlance).

Let users select the outputs from combo boxes so they can do it from LMMS and so that settings are persisted inside LMMS.

Implementation Details / Mockup

There is already a branch ready which implements this. 😉 For now it only supports output selection but there's already code to provide the same for inputs (see #7920). It looks as follows:

[michaelgregorius]

Here's how it looks with an alternative implementation using QToolButton which allows to select the input/output from a nested menu.

JackDriverInputOutputSelectionWithQToolButton.webm

The buttons look different to the other elements due to the global rules for QPushButton in the style sheet.