If username is not provided in properties, user is prompted for login repeatedly if login is invalid unless dialog is cancelled
See #53
When invalid login is provided in settings, error dialog is display instead of just silently reverting it.
See #53
Users can no longer override objects when selecting relation anchors from left to right.
See #55
Setting relation to None and overriding relations should now correctly remove the previous one if needed
See #55
Auxiliary objects for hyper-relations are now square instead of circle
When hypergraphs are enabled, the edge with index 0 is no longer discriminated by transparent color
Fix #54