some annoying bugs fixed

[zuzuf]

I fixed some annoying bugs today:
* the planned units preview now respect water depth (easier to put some underwater extractors )
* when planes were looking for a place to land, they didn't respond to orders until they found what they were looking for

the later was a tough one, but now it's fixed

[Balthazar]

Good news. Also would be nice, if ships and underwater units/structures doesn`t interfere each other and could move independetly.

[zuzuf]

This requires a better understanding of how ALL submarine units work ... I know some are flagged as submarines while others are just buildings that don't float but must be built on water, etc ...