rebased #2368 

All stair/slab nodes with parent nodes that are rotatable (wood and bricks)
are reverted to not having world-aligned textures, to fix the breakage of
rotated stair/slab appearence in worlds.

Update, and add missing documentation to, game_api.txt.

Remove flowers from coniferous forest.
Add 'dirt with coniferous litter' to farming mod overrides.

Currently all minetest_game PR's are failing travis since the
upstream luacheck now also warns about whitespace issues, and
there are a few of those in the code. This fixes all of them
so we can yet again rely on luacheck.

This removes the group overrides for dirt, dirt_with_grass and dirt_with_dry_grass which aren't needed and interfere with new grass spreading changes: https://github.com/minetest/minetest_game/pull/1286

Useful for searches and the mod profiler.

This doubles the fall height without damage to 11 nodes.

Remove line if set to the default of 'true'

- 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