feat(QML): add checks to ensure user run with a dedicated test profile

[acolombier]

This PR adds the safety mechanism we discussed during the monthly catchup.

It will prevent user from running the highly unstable QML interface, and allow us to continue with the fast iteration.

Here is the expected behaviour when running on QML:

• User starts with a new profile -> All good, starting
• User starts with an existing profile from a stable version -> User is presented with an error box and Mixxx stops
• User starts with an existing profile from a stable version but has given their consent to break things -> All good, starting with a critical message in the logs

In the case no 3, a log message and a key will be persisted in the user profile, which might be helpful to track future issues report on 2.x, tho user can easily remove it.

[Swiftb0y]

I really don't think this is super necessary, but I wasn't in the meeting this was agreed upon, so LGTM I guess?

[acolombier]

I really don't think this is super necessary, but I wasn't in the meeting this was agreed upon, so LGTM I guess?

To give some context, we've noticed some user are keen to try the QML instance as we are progressing, but the risk on corrupting existing user profile is large, since we are committing on a fast iterative set of many big changes for 3.0 (as opposite to a few small carefully measured and tested changes in 2.x).
This does shift the responsibility to our user being extra careful with their user profile, as this allows us to cut some corners and get rapid progress, but we still want to protect our user as much as we can. Does that make sense?

[Swiftb0y]

Yeah, I would've just been even more blunt with it and put no protections in place. IMO If a user is using a pre-alpha version, they are lucky they don't accidentally get their harddrive reformatted 😜

[acolombier]

I agree with you! But you need to remember that we are developing 3.0 in place, meaning that somebody installing 2.7-alpha will still be able to run it with --qml :)

[ywwg]

Just a couple of wording comments. I have not QAed the actual flow

[acolombier]

Thanks @ywwg - I amended the commit

[JoergAtGithub]

LGTM! Thank you!