Adds support for character project interruption (by waves) and for 'drowning' and 'undertow' which will both damage and kill the character if sustained. Tiles are now marked as being underwater when they are touched by water. There is still a bug where the underwater flag isn't being reset causing the characters to die if on a tile that has been submerged. Also need to reset build_time on project interrupt to prevent 'instabuild' on next structure