Allow loading custom sound config with '--sound-config' argument

[ronso0]

first step to resolve https://bugs.launchpad.net/mixxx/+bug/1516035

This allows loading a custom sound config via --sound-config config_file.xml. For now this is restricted to files inside the settings dir.
If the argument not added or the passed file is not readable, default soundconfig.xml is used.
Note: the given config file (command line or default) is used for all read/write operations in the current session, i.e. if you change your config those changes will be stored in your custom sound config.

Thus, the workflow currently goes like this:
Create sound profile

• connect audio devices and start Mixxx
• adjust sound devices and options as desired
• quit Mixxx
• copy soundconfig.xml and rename to something like soundconfig_DDJ-SB3_NIAudio8.xml

Load sound profile

• connect audio devices and start Mixxx
• start Mixxx with mixxx --sound-config soundconfig_DDJ-SB3_NIAudio8.xml

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I post-poned adding a profile selector to Preferences > Sound Hardware since that is more elaborate than I first thought. It's mainly getting the UX right, but also some flaws in current DlgPrefSound become obvious when dealing with profiles for the entire page (e.g. some settings are applied instantly).

[Swiftb0y]

Thanks for splitting into reviewable commits. lgtm for me so far.

[ronso0]

I think this is good enough to be mergd (if a second reviewer doesn't spot issues, of course).

The preferences / startup UI integration is a bit more elaborate and I'm unsure I find time to finish that soonish, so I'll do it another branch.

[Swiftb0y]

Thanks. LGTM. Though I still need to do a manual test to verify.

[ronso0]

Cool. I didn't do too many tests myself yet. With an arg config it shouldn't even matter if soundconfig.xml exists or not.

[ronso0]

Merge?

[Holzhaus]

Just a suggestion (not blocking merge or something), but if you can't specify full paths anyway, why not just dictate a naming scheme for the sake of shorter command lines?

For example, instead of:

mixxx --sound-config soundconfig_DDJ-SB3_NIAudio8.xml

we could use:

mixxx --sound-config DDJ-SB3_NIAudio8

And mixxx creates the filename from the name automatically?

[ronso0]

Sure, that makes sense.
Tbh I consider this PR a temporary solution, so I'd rather implement this in #4862. There, only the basenames are visible in the preferences so we'd (re)construct the file paths anyway.

[ronso0]

When reworking and simplifying #4862 it turned out that all the file(name) handling should rather happen in SoundManager, thus I suspend this one. Actually, dealing with filenames in general and in particular legacy soundconfig.xml is the bulk of the work :\

[ronso0]

I'm closing this. Sound config via command line is part of #4862 now

[ronso0]

Thanks for the review @Swiftb0y