(fix) Sound config: fix UX when applying config with missing/busy devices

[ronso0]

Fixes one cause of #13238 leaking EQ shelf controls / stuck overview on track load.

The scenario:

• start Mixxx without a previously configured sound I/O
• get warning that previous sound device for Main is n/a

  Mixxx was unable to open all the configured sound devices.

• click Reconfigure
• (no selection for Main), click Okay (or Apply + Okay)
• Mixxx proceeds (should actually throw the "No output devices" warning)

The issue:

There is a 'modified' flag in DlgPrefSound set on user changes, if false this will prevent re-applying the current config and the resulting sound pause when querying and re-setting the devices.
However, when the sound config is loaded in DlgPrefSound and devices are not found, the respective combobox will display 'None' even though that device is still set in the config. I.e. the visual state diverges from the config.
Clicking Apply would then not apply this new (visual) state since the 'modified' flag has not been set.

The fix:

Set the 'modified' flag if configured devices are not found.
Apply will then apply as desired.
(and, in case of missing Main output, Mixxx will throw the "No output devices" warning)

Leaking EQ shelf controls and stuck overview on track load is another story...

[ronso0]

Assigning this to 2.5-beta even though it's a 2.4 fix so we have it on the radar for the release.

[daschuer]

We have the 2.4.2 milestone and will do the last 2.4 release short before the 2.5.0 release. So let's use the 2.4.2 milestone for that.

[daschuer]

Thank you for the fix and the good documentation.