Changes done to the code in framework, comphelper and unotools,
in no particular order

- add an interaction handler dedicated to WebDAV
The stock interaction handler can be missing depending on the need of
the framework performing its tasks, so a dedicated handler is
provided, this one is always present.

- force opening of a WebDAV file.
A WebDAV file sould be open r/o even if explicitly requested to open
as r/w.
This is a limitation of current WebDAV implementation, not of the
standard.
This change is needed in order to reopen correctly a file as
requested by a 'Edit Mode' GUI command.

Change-Id: I5368fa2c0511f1630e6d6139c6a986d33aa19082
Reviewed-on: https://gerrit.libreoffice.org/17182Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Michael Meeks <michael.meeks@collabora.com>