Take over missing case from 'KDE5FilePicker::initialize' to
'Qt5FilePicker::initialize' and make
'Qt5FilePicker::addCustomControl' virtual, so that the
subclass's implementation is called in the 'initialize()'
method (in particular to avoid adding another autoextension
checkbox).

Drop the 'KDE5FilePicker::initialize' so that the
base class implementation is used, which now does the
same thing.

Add 'override' keyword in KDE5Filepicker.hxx.

Change-Id: Id151a4d4862af4275f6c21c6537a79c52e3ed513
Reviewed-on: https://gerrit.libreoffice.org/68059
Tested-by: Jenkins
Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>