Implement "textarea", a multi-line input field in formspec

<img src=http://i.imgur.com/pTY7z.png>

As the title says, this is a textarea element for formspec.
For instance, this can be used by the microcontroller in mesecons.
Use it just like a positioned field.

You may want to download a reference mod in order to test the change: http://mesecons.net/random/test.zip

test:test1 is a block that can be used to test the new textarea.

(test:test2 is basically the mesecon command block's formspec from that proofs the old fields still work)