pathbar: Handle webdav where is the root is a path (backport to 3.24)

Our webdav server has a root which is davs://mynextcloud/remote.php/webdav When once creates a GFile out of or out of a subdirectory, and one call g_file_get_parent(), it recurses too far up and try to query davs://mynextcloud/remote.php which fails, resulting in a broken pathbar.

To fix that, before querying the metadata of each element of the path, I query the "enclosing mount", then use it's root to compare the GFile against.

This is a backport of !5830 (merged) to 3.24 and it also includes fix !5982 (merged)