Add initial support for Minetest schematic API

This patch adds support for creating and placing schematic files using the new schematic API.
Three commands were added:

- /mtschemcreate <filename>

Creates a Minetest schematic file of the nodes on the map in the box from pos1 to pos2, and saves to filename
- /mtschemplace <filename>

Places Minetest schematic from filename onto pos1 of the map.
- /mtschemprob <start/finish/get>

Before running /mtschemcreate, probability values of nodes inside the box can be set.  /mtschemprob start begins the selection process, and node punches thereafter open a formspec with a text entry box to enter the probability value for that node from 0 to 255.  /mtschemprob finish exits this selection mode.  /mtschemprob get displays the probabilities of the nodes that had been modified, pending schematic creation.