Store last click's coordinate

[remikey]

A user expressed a use-case in which he wanted to be able to click somewhere on the Topology and be able to later reference the corresponding location.

Although he eventually tackled his project differently, it would be nice to help the user with "standard tools" to:

• detect that this empty location has been clicked (middle click?)
• store a location in the core (not Swing specific) for later use