Tweak remove/hide track confirm message to match confirm exit message

[ninomp]

This is seeking to fix https://bugs.launchpad.net/mixxx/+bug/1947992.

This is a follow-up of #4330.

[foss-]

Testing 2.4-alpha-1034-g91bcb96ec6 (HEAD) from https://github.com/mixxxdj/mixxx/actions/runs/1608937872 does not resolve the UI issue reported in https://bugs.launchpad.net/mixxx/+bug/1947992:

[ronso0]

@foss- Is the button appearance of the (hide message) identical for all skins?
I'm asking because with this PR the only difference between the "Close Mixxx?" and the "Confirm Track Hide" message is the parent widget: "Close Mixxx?" child of mixxxmainwindow, "Confirm Track Hide" child of WTrackTableView. The latter may have a stylesheet applied, with snippets like WLibrary QPushButton ....

[foss-]

This is more divers than anticipated.

Deere (all caps):

Latenight:

Shade (tiny buttons):

Tango (expected):

[ronso0]

Okay, so this needs to be addressed by changing the stylesheets. I'll take a look.

This PR LGTM

[ronso0]

@ninomp Feel free to pick the stylesheet fix 49d10e5
(I'm too lazy to open a PR atm :)

The dialog now looks good (system style).

[ronso0]

@foss- Can you confirm the dialogs have the default system style now?

[foss-]

Confirming this is solved, dialog looks good in all skins.

[ronso0]

Great, thank you @ninomp for fixing and @foss- for testing :)

[ninomp]

@foss- Thank you for testing.
@ronso0 Thank you for assistance and review.