When opening a file that triggers Dialogs (e.g. cannot
read/repair/FileType) the Frame from which it was
initialized gets blocked. This irritates quite some
people. Changed this to a non-modal Dialog so that
the user can continue to work with all opened docs,
open new ones, close and print/PDF/export these.

Change-Id: I048d3de3369527cec20d26396b87439254764b8a
Reviewed-on: https://gerrit.libreoffice.org/41534Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Armin Le Grand <Armin.Le.Grand@cib.de>