feat: Add shallow water in editor

[dvincent56]

• Use unused shallow assets
• Add shallow tile in editor under water menu

[crudelios]

I'm not sure you need to increase the savegame version as well.

[Johannes-callidus]

I'm very sure you have to increase the save version as well because terrain grid is saved in both scenario and save game. And otherwise the game will try to load a terrain type which isn't yet in the game. Although this might not be that important since it's a bitflag enum so the game might just ignore the additional terrain bit I think it's good practise at least.