Add schematic decoration force placement flag

Add find_surface_nodes_in_area LUA call which permit to only get the nodes which touch air. This permit to massively improve performance for mods like plantlife

Rebased by Zeno- (conflict in lua_api.txt)

Client now informs about incompatible servers from the list, this permits to prevent the protocol movements.
Server announces its supported protocol versions to master server

Also add a Lua API and chatcommand for this

Lua-api.txt: Document that paramtype='light' results in light propagation with attenuation and is essential for a light source to spread it's light

Just Markdown, no variant of it. File name intentionally
not changed to “lua_api.md” because of fear to break references
to that file. The file can still be renamed later.

Also rewrite mapgen registration for static initialization

Add minetest.get_mapgen_params()
Deprecate minetest.register_on_mapgen_init()

Also set default height_min/height_max to -31000 and 31000,
respectively, for ore and biomes

If CMake is run and Doxygen is found, add a make target called "doc",
which builds API documention in doc/html. This target is not included
in the default "all" target, you have to explicitly run "make doc" to
generate the documentation.

If graphviz is installed, in particular, if the "dot" binary is found,
doxygen is configured to generate various kinds of diagrams. Note that
due to this, the first run of doxygen can take a while.

Add support for notify-on-decoration
Clean up mapgen constructors
Clean up mapgen.cpp code style somewhat
Remove trailing whitespace from some files

Changes:
  * Add extra documentation for remove_item.

    This isn't as silly as it sounds: iterate a list containing items with
    unique metadata each, and remove_item the first one you find, placing
    into a different - fine, except the remove will invariably remove the
    wrong one, leading to items being duplicated and others destroyed.

    Arguably it's a bug, and Inventory::removeItem should actually remove
    the item you tell it to. (i.e. if the passed ItemStack has metadata,
    match it). But it seems reasonable to just document the behaviour
    more clearly.

 * Improve string_to_pos documentation.
 * Clarify dig_node documentation (return value).
 * Better on_step documentation.
 * get_nodemeta -> get_meta.
 * Other minor fixes.