Qt5 hide more Qt5FilePicker details

* Make most members private
* Drop some unused members
* Rename some single-char variables for easier reading
* Spread some constness
* Finalize the KF5FilePicker class
* Test the native picker setting of the QFilePicker, instead of an
  additional bool member

This is just some refactoring. While not strictly needed, it helps
keeping the following patches fixing tdf#129071 more compact.

Change-Id: I15ffe4de848a9498d7f61f99bcf031257da7cb08
Reviewed-on: https://gerrit.libreoffice.org/84456
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>

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