Improve foundation placing

[Itms]

Since r15612, some problems arise with foundations and the way uncommitted foundations (HP = 0) are handled. Miraging them can lead to strange behaviors.

Moreover, one can cheat by hovering a preview of a building over explored territory. If a new building has been placed, or if a real entity has been destroyed, the actual entities will influence the placement restrictions so it is possible to know what is hidden.

A solution would be to create foundations as mirages, and to make them real entities only once they are committed. Obstruction problems should be handled then.