Optionally show context menu when right-mouse button is clicked on top of entity.

Originally from FS#1111: "Furthermore, the two named applications show that it's easily possible to combine dragging (mouse pressed) and context menu (mouse clicked) on the same mouse button.
Providing a context menu would increase the usability especially for people used to Windows default applications.
I.e. deleting, copying, creating polylines from segments, breaking up blocks/polylines would be functions I'd find useful in a context menu."