Sound Hardware preferences should auto-select next free channel

When configuring sound hardware in the preferences, Mixxx currently always assigns the first channel of a soundcard to every input or output instead of using the next free available channel.

For example, if you want to configure the main and headphones output, the workflow is the following

1. Select soundcard A in the main output's soundcard combobox. The channel combobox automatically picks channels 1-2.
2. Select soundcard A in the headphone output's soundcard combobox. The channel combobox automatically picks channels 1-2. (the configuration is now invalid because both outputs use the same channels)
3. Select channels 3-4 in the headphone output's channel combobox.

Mixxx should automatically pick channels 3-4 in step 2, because channels 1-2 are already in use. That would make step 3 unnecessary.