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.