MixxxApplication: Only log opaque pointer after notify

[fwcd]

As an addendum to #13900, QEvent::Timer seems to be another event type where a dangling pointer could come up (the screenshot includes some more logging which I've removed again):

Since the target pointer could be dangling for any event point, this PR takes the safer road by only logging the opaque pointer after the notify call.

[JoergAtGithub]

I think that this could happen with any event type - but QEvent::DeferredDelete has the speciality, that it always happen (if we hit the time limit).
As I expressed already in #13889 (comment) I see only two proper fixes:

1. Cache the object informantion before calling notify, which will affect the general performance in developer mode
2. Remove the printout of the target object information and just print the information about the event type

[fwcd]

IMO not printing the target (or only the hex pointer) would be the safe option for now if we don't know whether the pointer might be dangling. Maybe we could add another verbosity or log level for caching the object info and logging it?

[JoergAtGithub]

@ronso0 has already opened a related PR #13909, which removes the only known case to use --developer for non-development.

[fwcd]

Yeah, I believe we should still fix this though, since non-deterministic crashes shouldn't happen, even in developer mode.