fix app/window icons

[ronso0]

• fix regression introduced by 564df77, all dialogs (except warnings) now use the Mixxx icon
  (not 100% sure I caught all, and I didn't figure why the icon is implicitely inherited, for example by the Replace Hotcue Color dialog)
• make About dialog reachable via Alt+Tab by not parenting it to MixxxMainWindow
  (this was a bit annoying until now, i.e. when trying to jump back and forth between About Mixxx and web browser to report bugs)

[JoergAtGithub]

The regression is in the 2.3.1 branch, shouldn't the fix go into 2.3.1 too?

[ronso0]

I read this

while #4204 actually went into 2.3

will rebase.

[ronso0]

what's left to do @Be-ing ?

[uklotzde]

I will perform a test and then merge. Sorry for the delay.

[uklotzde]

I guess this cannot be tested properly without creating an installer and actually installing the broken, current version. I am too lazy spending time on all this. But the changes seem reasonable.

@Be-ing Merge anyway?

[Be-ing]

thanks