Arraying pads in footprint editor should not assign pad/pin numbers for aperture pads (lp:#1781760)

[Original report](https://bugs.launchpad.net/bugs/1781760) created by **Andy Peters (devel-latke)**
```
This issue seems to be the standard operation as of the 14 July 2018 RC3
for Version 5.

The footprint editor allows the user to easily replicate pads in an array,
which is ideal for creating BGA, PGA and similar packages. It also
automatically assigns pin/pad numbers to each pin.

But this pad-number-assignment behavior is not good when the user wants to
create an array of aperture pads. Aperture pads are those without a copper
layer assigned. They are useful, for example, when creating custom solder
paste mask openings on a package which has an exposed thermal pad. Since
these pads don't have copper connectivity, they don't, and indeed cannot,
have pin numbers (nor can they have nets).

Also, the pin number of a pad with no copper layer assigned does not appear
in the pad, so even if a pin number is assigned, it's not visible to the
user.

My recent experience is this. I had a board made with a QFN w/exposed
package chip and I ordered a stencil for it. The footprint I used for this
chip had the solder paste mask checked for the entire exposed pad, so the
stencil had a big square for the exposed pad, which is not good. I wanted
to order a new stencil with the recommended 3x3 array of 1.25 mm square
exposed areas on 1.8 mm pitch, so I edited the footprint in place. I edited
the pad and unchecked the F.Paste layer for it. I then created a 1.25 mm
square pad with no copper and only the F.Paste layer enabled. I made sure
that the pad number field was blank, as well as the net name field. I
created the pad array, and updated the PCB with this footprint.

The PCB ratsnest showed that some connections on the chip needed to be
made, and DRC complained about unconnected traces/pads. The ratsnest showed
that I needed to make connections to the paste-mask-only pads I just added.

Again, those pads did not show any pin number, so it was somewhat baffling
why pcbnew wanted to me make a connection to a pad with no copper, which
meant no way to actually make that connection.

In pcbnew, I looked at the aperture pads' Properties, and saw that they had
pin numbers assigned, as such:

+--+--+--+
|33|23|13|
+--+--+--+
|32|22|12|
+--+--+--+
|31|21|11|
+--+--+--+

and the net name of each was the same as the real pin of the same number.
So that explained why the connectivity check thought the aperture pads
needed to be connected. But since the pads have no copper, the connection
is impossible.

So, suggestions:

a) In the footprint editor, it should never be possible to assign pad
numbers and net names to aperture pads. That is, if no copper layer is
enabled for the pads, the pad number and net name edit boxes should be
grayed out/disabled and any text in them should be deleted.

b) The footprint editor array function should never assign pad numbers to
aperture pads.

Doing so will prevent pcbnew from assigning a net name to a pad with no
connectivity, and that'll keep the connectivity check happy.
```
Original tags: array

Story points: 8.0