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

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

Swing the camera down when the player lands on the ground, based on the velocity the surface is hit with.

Switch the license to be LGPLv2/later, with small parts still remaining as GPLv2/later, by agreement of major contributors

The huge item definition and item namespace unification patch (itemdef), see http://c55.me/minetest/wiki/doku.php?id=changes:itemdef

inventorycube: use all three specified textures; also moved mesh creation / modification functions to mesh.cpp; in lua, inventorycube is now called minetest.inventorycube