Add (experimental) support for topbottom as well as sidebyside 3d mode

Minor coding style fixes

Add support for (configurable) multiline hotbar
Improved screensize handling
Add userdefined gui scale by BlockMen

Parallax mapping with slope information.
Overriding normal maps.

Remove flagmask field from set_mapgen_params table
Add small bits of needed documentation

Add checks for invalid user input for important settings
Rename Settings::tryGet* to Settings::get*NoEx for consistency

MapgenParams is no longer a polymorphic class, eliminating the need for messy and bug-prone reallocations.
Separation between the common and mapgen-specific parameters is now strongly defined.
Mapgen parameters objects are now properly encapsulated within the proper subsystems.

The commit didn't work because the blocks weren't loaded yet.

This reverts commit 22dbbf0a.

Conflicts:
	minetest.conf.example

Move most of the media-related code in client.cpp into a new class
ClientMediaDownloader (clientmedia.cpp, clientmedia.h). Among other
things, this class does the following things:

- Download [remote_server][sha1] instead of [remote_server][name]. This
is to support servers that provide the same file name with different
contents.
- Initially fetch [remote_server]index.mth. This file should follow the
Minetest Hashset format (currently version 1) and contain a list of SHA1
hashes that exist on the server.
- The list of needed SHA1s is uploaded (via HTTP POST) when index.mth is
requested, so servers can optionally narrow down the list to the needs
of the client.
- If index.mth is missing (HTTP response code 404), we enter compat mode,
fetching [remote_server][name] as before this commit.
- remote_server can now contain multiple servers, separated by commas.
The downloader code attempts to split requests between the different
servers, as permitted by each server's index.mth. If one server claims
to have a file but actually doesn't (or something fails), we ask a
different server that also claims to have it.
- As before, when none of the remote servers provide a particular
file, we download it via the conventional method, i.e. using
the minetest protocol: TOSERVER_REQUEST_MEDIA / TOCLIENT_MEDIA.
- Bugfix: Every downloaded file's SHA1 is now verified against the SHA1
announced by the minetest server (before loading it and inserting it
into the file cache).
- Bugfix: Only send TOSERVER_RECEIVED_MEDIA when we actually have all
media. This should fix #863.

Add curl_parallel_limit setting that will replace media_fetch_threads in
a later commit.

Fix a typo in MutexedQueue::pop_back() that made it impossible to compile
code that used this function. (Noticed this while implementing httpfetch.)

Change large amount of objects to be configurable via a setting in minetest.conf (max_objects_per_block).