Expose sample rate in settings dialog

[zynskeywolf]

Adds basic ability to manually set the preferred sample rate used by the audio backend. Basic and preferred because the list box is not actually populated based on the backend's report yet.

Fixes #4607

[sakertooth]

Just tested, everything seems to be working as expected.

[PhysSong]

The strings for audio interface name could be changed due to translations and future code changes. According to line 493 and 536-549, the value will be AudioDeviceSetupWidget::tr(AudioJack::name()).
I think you should update the combo box based on the AudioDeviceSetupWidget child class to be selected.

[zynskeywolf]

Good point, but as side question, do we really have a good reason to localise the backend names? They are their Names after all.

[DomClark]

There are specific configuration widgets for each backend - would it be better to put the configuration option in those, where it can be adapted to each specific backend (e.g. omitted if the backend doesn't support a choice of sample rate, or be populated with the available rates)?

[zynskeywolf]

There are specific configuration widgets for each backend - would it be better to put the configuration option in those, where it can be adapted to each specific backend (e.g. omitted if the backend doesn't support a choice of sample rate, or be populated with the available rates)?

My motivation to not put it in there was the fact that it does not actually set the sample rate of the audio backend directly but rather it's a preferred setting for the lmms engine which then TRIES to preferably open the device using the specified frequency IF possible. I intend this PR only as a quick workaround until the audio backend mess actually gets sorted out. (Thinking of the misterious "HQ mode" option and the like)
Putting it in the backed-specific widget would suggest that the device itself will be opened with the specified setting, but for that to actually happen we first need to have the ability to query the supported settings.