Add TO-220 horizontal with tab pad connected as pin #4

I'm not sure if I'm not bending the rules too much, but this one passes validity checks and seems to be rendered properly in both PDF and Gerber files.

This is a TO-220 footprint which allows to connect the back pad as pin #4. The pad is actually represented by two instances:

* a big front pad where the back of component is going to be placed on the board,
* a small pad on the other side, around the mounting hole (actually thought about it more as a sort of clearance than actual electrical connection through the screw, but it allows for both.

I'm also submitting a couple of MOSFETs with the fourth pin present to the kicad-library repository. They seem to be working together pretty well, at least for my small 2-layer projects.