New fishing content [WIP]

This is the initial PR.
Some more fishes (and some z-fishes) were created and they are added to the RIVER_GROUP spawnlist.
As such, they spawn normally. ( i expect the spawn chances to need lots of tweaking)

TODO:
1) Make the fishing action to fish from the existing river critters.
2) Find ways to adjust the fish population (and the fish spawns accordingly) in response to fishing.