Allow specifying a protocol in the host field when mounting another
ownCloud instance. Note that this was already possible with the WebDAV
config but this bug made it inconsistent.
To make it possible to use the short ownCloud URL (without specifying
webdav.php or remote.php), a new backend is available for ownCloud.
The user must specify the host + context path in the "Url" field (which
is mapped to the "host" parameter) and the subdir to mount in the "Root"
field.
This is to prevent confusion because some users forget to append
webdav.php or remote.php to the WebDAV URL.
Fixes#4923
