Enable colored Luacheck output on Travis CI

see #2400, also removed a now unused alias

Code simplification by combining the on_place functions.

sfinv: Add lists above the content to support listrings in content.
Reuse sfinv inventory lists for creative.

Icesheet ice dungeons now have ice stairs.
Remove some now-unnecessary sandstone mapgen aliases, one
remains as it is required by the dungeon loot mod.
Re-arrange mapgen aliases to separate those needed for mgv6.

This schematic is used for sapling-grown large cacti, so should not
destroy the sand node below, as sand nodes are in short supply in
some situations.
Like trees, only force-place the node that replaces the sapling.

The author is used to track packages for updating, so it should match.

No longer use the hardcoded engine cave liquids.
Water only in '_ocean' biomes, water and lava in '_under' biomes.

Previously, opening a trapdoor caused the side textures to flip.

Fix the incorrect textures.
Also add a texture transform to a tile of the open trapdoor, such
that the closed trapdoor sides use the lower part of the texture
and the open trapdoor sides use the higher part.

Clean up some codestyle issues.

In preparation for biome-defined: dungeon materials, cave liquids,
stone type, ores, decorations.
'_ocean' biomes now extend to y = -255 to be deeper than default
mgv5 oceans, and to create 'shallow underground biomes'.
Remove unnecessary biome lists for gravel and silver sand blob ores,
as those are already defined to only appear in default:stone.

fixes #2228

This is a rebased and slightly rewritten version of #2125.

This also removes the deprecated `sudo: false` option, as Travis CI
is phasing out the container-based infrastructure.

Footstep sounds as sand or cubic coral nodes.
Dig and dug sounds as leaves but much lower gain to be suitable for
underwater.