Feature/sinkwreck

Issue #19 

* Hover, ship and air units sink better now in water (using projectile instead of slider)
* Wrecks real positions are now on the seabottom, making it easier to target for reclaiming.
* Removed alot of duplicate code in defaultunits.lua regarding sinking and wreckage

Death, explosion and sink effects could still need tuning. Wasn't very good earlier either