Dropped support for punching doors to open them - only right-click now.
Moved gates out of fences and into doors file, renamed that file to
doors_and_gates.lua.
Changed gates to also use right click to open, and in the process
completely rewrote how gates are defined and managed; this necessitated
creating some duplicate textures. Some of them are blank, and texture
pack authors may find the extra filenames useful anyway.
Node names for all gates have changed, aliases are provided for backward
compatibility.
