show preferences when configured skin failed to load

[ronso0]

WIP
Addressing a 9 year old TODO
Actually this should happen only with a broken skin from the user directory.
Though, in that case the user has no chance to get back to a working state from within the GUI.
(only by editing mixxx.cfg)

Candidate for 2.3, though when no one wants to squeeze that in right now it can wait for 2.3.1 We got along without it until now.
rebase after #3839 is merged

[Holzhaus]

Alternative: We could switch back to the default skin instead of opening preferences. This saves some work for the user.

[Swiftb0y]

We'd have to inform the user of that behavior with a popup or similar, otherwise they might think its a bug.

[Swiftb0y]

The ideal would probably to have a similar dialog like we have for misconfigured audio settings. So they have the choice between "Exit", "Reconfigure" or "Continue with Default".

[ronso0]

...which requires a more elaborate dialog instead of the standard Ok / Cancel // Yes / No
https://github.com/mixxxdj/mixxx/pull/3212/files#diff-848ff12a6bbb8c39f28e2a6be33ca2bbbe31f53e6ba550b2f6df7b8bdfaecf1fR1666

(this fix is adopted from the unconfigured Mic/Passthrough situation)
Considering the TODO has been sitting there for 9 years we may merge this quick fix now to ease this situation for a few newbie skin developers -- and polish the UX in a follow-up (2.3.1). Or --regardless of 2.3-- merge when dialog UX is perfect.

I don't mind. I could deal with it for years. Actually this fix is only for convenience, it neither affects skin developers (as they know what files they were messing with), nor users downloading modded skins (those skins should work to a degree, at least skin.xml should -which is the only file to be corrupt to make skinloader fail).

[Swiftb0y]

Yes we can with the current feature otherwise this PR will just suffer from feature creep again.

[ronso0]

btw another place where we should trigger the same popup
https://github.com/mixxxdj/mixxx/blob/2.3/src/mixxx.cpp#L1553-L1568
to reduce duplication we should probably refactor rebootMixxxView() to be used in initialize()

[ronso0]

This is fixed in another way by #3891

[Holzhaus]

This is fixed in another way by #3891

Sorry, I somehow forgot about this PR. Didn't want to make your work obsolete.

[ronso0]

don't worry. Currently I have neither the motivation nor the time to get this small fix into a mergable state.
it was just a drive-by fix anyway