Issue #811 - new gravel texture needed.

This texture was Gambits' PixelBOX gravel light texture. Gambit
posted that his texture pack is WTFPL:
- https://forum.minetest.net/viewtopic.php?f=4&t=4990&start=50#p141196

I've made significant modifications to this texture:
- slightly rotated and rolled some sections of pixels
- minor burn/dodge some pixels to keep high contrast
- removed lineair repeating effects
- etc.

Attribution is added back to Gambit. Thanks.

Adds a birch-like tree to the default_game. Aspen was chosen on
purpose instead of birch, as several birch tree mods already exist
and choosing a different name avoids any conflicts.

Schematics were made for both normal and sapling version, assuring
saplings will not be able to grief. The shape of the Aspen is "fanning
out at the top" and provides an easy tree to walk under, but still a
somewhat thick cover. The Aspen trunk is 4 to 6 blocks tall, with up
to three layers of leaves on top, making it slightly taller than an
Apple tree, but shorter than a Pine tree, which provides a good blend.

Textures were painted from scratch, except tree_top and _wood
variants, which are color modified versions of the pine_wood
variants. Appropriate licenses have been chosen for the new textures.

The leaf texture is light enough to contrast other trees, but dark
enough to offset against our light default grass color. The leaves
are drawn in the typical minetest default fashion with plenty of
transparancy, but enough definition to suggest that you're seeing
something that looks like leaves. The placement of leaves in the
schematic also suggests the top of the tree is sparse and you can
see the sky through the leaves.

Sapling texture is both traditional and different, with lush green
leaves and a well-defined stem, but slightly stick-like and skinny,
as these plants tend to grow up first, then out.

Add fallen Aspen logs. We make these logs a minimum of 2 blocks long,
and up to 3. This allows us to make these logs a place where both
red and brown mushrooms can be found, to these may be attractive to
players. However, the spawn rate for these has been reduced a lot
compared to the other logs, to account for the scarcity of Aspen.

Add stairs, slabs for these wood types as well.

Mapgen will place these trees in deciduous forests only, but in
a way that the biome is a range between entirely Apple trees, and
mostly entirely Aspen trees, with a bias to Apple trees. To make
fallen logs somewhat correlated with trees, we modify the planting
of Apple trees and logs to use perlin noise and not fill ratio,
otherwise you'd always end up with Apple logs in Aspen tree areas,
which would be suspicious. There still is a bit of a mix.

Since written books are quite different from empty books, the
visual clue that they are different items is really needed in-game.

I've added a "clasp" or "belt-with-buckle" like element to the
png from default_book, keeping them very similar but also
immediately recgnizably different.

I added the new texture to blockmen's license list since it's
obviously derivative of his work.

The PNG was run through a minimizer/optimizer to save space.

Rename in game.conf and documentation
Update game_api.txt documentation for bucket API and tree functions
Fix tab, space and comment formatting in game_api.txt
Rename in mod READMEs

- Mese glows
- Glass + Mese = Meselamp
- New mese mineral texture

I forgot frame range definitions need to be per-model, since each model could have its own animations. Specify current ones as being player.x's

Add death animation, and correct many mesh issues I didn't notice last time

Remove segmentation of limbs, because I don't like how the arms appeared to shrink during some deformations. This makes the player mesh exactly like the one in MineCraft, and gives people more reasons to complain that we're copying it :P

Rename player to character, to avoid conflicting with the default player sprite

New skin by Jordach

Add licenses to readme for the model and skin

Fix bad mirroring of left arm for player model