Replace C++ mainmenu by formspec powered one

builtin/gamemgr.lua

builtin/mainmenu.lua

builtin/mainmenu_helper.lua

builtin/modmgr.lua

builtin/modstore.lua

doc/lua_api.txt

doc/menu_lua_api.txt

minetest.conf.example

src/CMakeLists.txt

src/convert_json.cpp

src/convert_json.h

src/defaultsettings.cpp

src/filesys.cpp

src/filesys.h

src/game.cpp

src/guiConfigureWorld.cpp

src/guiConfigureWorld.h

src/guiConfirmMenu.cpp

src/guiCreateWorld.cpp

src/guiCreateWorld.h