New fence texture
Make farming more flexible

- API {farming.register_hoe(), farming.register_plant()}
- Fertilities -> Plant only grow on soil with a fitting fertility, e.g. Wheat only grows on grassland, while cotton grows in deserts and grassland)
- New soil: Desert Sand
- Place seeds instead of plants
Merge remote-tracking branch 'minetest_next/master'

Conflicts:
	mods/default/textures/default_fence.png
	mods/farming/init.lua