Fix issue (GH6290) in new DialogDisplayerImpl behaviour

[neilcsmith-net]

Attempt to fix issue #6290 in new DialogDisplayerImpl behaviour introduced in #5989 and #6216.

Possible alternative to reverting in #6291

[neilcsmith-net]

Force pushed (again) because of failing test. Actually realised the test had been updated as part of #5989 and the update here behaves like the previous NetBeans code did. Putting back the old test line.

[errael]

I still don't understand the whole leaf thing, but since I haven't tried I don't feel to bad. This certainly seems to avoid parenting to a Window. I'm good with it (except for the initBound()).

Took a news break while reviewing, came across https://www.cnn.com/videos/us/2023/08/02/boston-police-officer-falls-down-slide-cprog-mh-orig.cnn which somehow resonated.

[mbien]

tested the last dev build of this PR and it seems to be working fine - without exceptions in the log.

I did also notice that the save/discard dialog which pops up when #4792 is used is now centered on the main window, which is better since it used the popup as parent in NB 18 believe.

[neilcsmith-net]

Thanks @mbien Also confirmation from the bug reporter on the dev build. Let's merge and test in rc4.