Only create SoundManager in main menu if USE_SOUND is true
Remove common from CMakeLists.txt, README.txt, lua-api.txt and buildwin.sh
Remove survival and build from CMakeLists.txt
New modsystem
Mods are placed in $path_<user/share>/mods
They can be enabled per world in world.mt or the configure world window
Fix regression of world loading caused by 81c863ac
Remove no virtual dtor warnings, make MapgenParams contain actual NoiseParams
Install mods/mods_here.txt instead of mods/minetest/mods_here.txt