Add net ties

Adding net tie footprints as discussed at https://github.com/KiCad/kicad-library/pull/1472.

This is for discussion purposes right now. There should not be merged as-is. I realize the descriptions are bad, line endings are not Unix, no connecting copper, etc. I would like to discuss the following (and any other points):

1. I chose Net-Tie_{pin count}_{SMD|THT}_{pad diameter}. Do we need to indicate pad shape?
2. THT pads are circular.
3. They are all virtual components.
4. They don't have F.Fab ref des.
5. All pads are covered with mask.
6. A zone will be added on front (SMD) or front and back (THT).
7. SMD pads have no paste which seems fine to me.

I'm planning to add 3- and 4-pin footprints in the same pad sizes once we decide on the above, giving 12 footprints total.

Image: