Double click structure should make it a primary structure

[stefanhendriks]

Instead of pressing P or right mouse click, change this into double click.

Do note: There is no such thing as double click in the game yet. So that requires some logic in the mouse observer to emit new 'double click' events.

[stefanhendriks]

Once there is a double click principle, we can also do https://github.com/stefanhendriks/Dune-II---The-Maker/issues/318