Formspec
--------
Formspec defines a menu. Currently not much else than inventories are
supported. It is a string, with a somewhat strange format.

Spaces and newlines can be inserted between the blocks, as is used in the
examples.

Examples:
- Chest:
    size[8,9]
    list[current_name;main;0,0;8,4;]
    list[current_player;main;0,5;8,4;]
- Furnace:
    size[8,9]
    list[current_name;fuel;2,3;1,1;]
    list[current_name;src;2,1;1,1;]
    list[current_name;dst;5,1;2,2;]
    list[current_player;main;0,5;8,4;]

Elements:

size[<W>,<H>]
^ Define the size of the menu in inventory slots
^ deprecated: invsize[<W>,<H>;]

list[<inventory location>;<list name>;<X>,<Y>;<W>,<H>;]
^ Show an inventory list

image[<X>,<Y>;<W>,<H>;<texture name>]
^ Show an image
^ Position and size units are inventory slots

field[<X>,<Y>;<W>,<H>;<name>;<label>;<default>]
^ Textual field; will be sent to server when a button is clicked
^ x and y position the field relative to the top left of the menu
^ w and h are the size of the field
^ fields are a set height, but will be vertically centred on h
^ Position and size units are inventory slots
^ name is the name of the field as returned in fields to on_receive_fields
^ label, if not blank, will be text printed on the top left above the field
^ default is the default value of the field
  ^ default may contain variable references such as '${text}' which
    will fill the value from the metadata value 'text'
	^ Note: no extra text or more than a single variable is supported ATM.

field[<name>;<label>;<default>]
^ as above but without position/size units
^ special field for creating simple forms, such as sign text input
^ must be used without a size[] element
^ a 'Write It' button will be added automatically

label[<X>,<Y>;<label>]
^ x and y work as per field
^ label is the text on the label
^ Position and size units are inventory slots

button[<X>,<Y>;<W>,<H>;<name>;<label>]
^ Clickable button. When clicked, fields will be sent.
^ x, y and name work as per field
^ w and h are the size of the button
^ label is the text on the button
^ Position and size units are inventory slots

image_button[<X>,<Y>;<W>,<H>;<texture name>;<name>;<label>]
^ x, y, w, h, and name work as per button
^ image is the filename of an image
^ Position and size units are inventory slots

button_exit[<X>,<Y>;<W>,<H>;<name>;<label>]
^ When clicked, fields will be sent and the form will quit.

image_button_exit[<X>,<Y>;<W>,<H>;<texture name>;<name>;<label>]
^ When clicked, fields will be sent and the form will quit.

Inventory location:
- "current_name": Selected node metadata
- "current_player": Player to whom the menu is shown
- "player:<name>": Any player
- "nodemeta:<X>,<Y>,<Z>": Any node metadata