Turn existing functions in Qt5FilePicker into protected
static class methods and reuse them in KDE5FilePicker.

The qt5 implementation covers everything that kde5 had,
and a little more (e.g. covers case 'ControlActions::DELETE_ITEM'
in 'handleSetListValue' as well).

Change-Id: I9673b20e3313c0628a76f0d94e018327f4af8523
Reviewed-on: https://gerrit.libreoffice.org/68054
Tested-by: Jenkins
Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>