Allow customisation of connectors

[MattIPv4]

Currently, the connectors in the app are automatically given a specific style based on the determined connection type.

Whilst this is technically accurate and is great default behaviour, we should allow the user to change the style of the connector line if that is what they want to do. We should offer all the current connector styles currently in the app, as well as any others featured in the diagram kit that we don't currently have.

Further to this, we should also allow the user to optionally enable arrow heads at either end of a connector line.