LO already supports http, https, webdav and webdavs through the
WebDAVContentProvider. Ftp is supported via FTPContentProvider
and then finally we have the GIOContentProvider that can potentially
support SMB, if the dependencies for that are met.

We now configure the KDE file dialog to allow these remote protocols.
Note that this filtering depends on https://phabricator.kde.org/D10024
and https://phabricator.kde.org/D10025 to have any effect.

Then we rewrite the URLs we receive from KIO to a format that is
supported by LO. Most notably, we prepend `vnd.sun.star.` to the
webdav URL schemes, such that they get picked up by the
WebDAVContentProvider. Then finally, we clear the username from
the smb:// URLs we get from KIO, as that prevents GIO from opening
them.

In all cases, the user will get prompted a second time for the
credentials required to access the remote resource. This is
unfortunate, but better than nothing. In the future, we may solve
this issue through either a separate KIO UCP or by getting support
for the FDO Secret Service specification in KWallet.

Change-Id: I91df28434b115639c2698968e2a672b3320bf8e2
Reviewed-on: https://gerrit.libreoffice.org/48350Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>