In order to use GetSelectedFiles (in XFilePicker2) instead of GetFiles (in XFilePicker),
here are some api changes

before:
XFilePicker3 inherits from XFilePicker
XFilePicker2 is not a published interface

after:
XFilePicker3 inherits from XFilePicker2
XFilePicker2 is a published interface

+ adapt Uno Implementations

Change-Id: If44afaa7236f08bc2b814f91eda5bfad333dd799
Reviewed-on: https://gerrit.libreoffice.org/17068Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Stephan Bergmann <sbergman@redhat.com>