Create custom layers according to GerberX2 FileFunction spec

[Dan Sheadel]

Currently, Kicad only supports usage of a small number of layers, particularly the 4 User/Eco layers. Because these layers are not intended for fabrication use, creating a custom fabrication/technical layer means you must abuse these layers, and manipulate gerbers out-of-tool in order to match fab requirements for layer names for fabrication.

Allowing creating of custom layers according to the Gerber X2 spec would give a couple advantages here:
- Most needed layers can be done in tool (vscore, via fill, etc)
- Can be set up in a standard compliant way (because it's off a standard)
- Few engineering decisions: Gerber X2 has done the hard work of figuring out the names and text.
- The widgets and infrastructure to implement this are mostly in place (Layer Setup has user-input for Names, and a Type dropdown). It just needs to be extended with additional types following the X2 spec

The exception to the existing infrastructure would be FileFunction,Other, which requires a mandatory type description. So, this would require either a supplemental text field, or to use the Name as the type description.

An example using V-scoring would help clarify the utility. Currently, to represent this, you would need to
- Draw the V-scoring on User1 (or other layer)
- Generate gerbers
- Indicate to the fabricator to use Eco1.gbr as V-score, or manipulate the file to detect it as such.
- Any time the version is updated, the gerber data MUST be appropriately updated every time, increasing likelyhoood of failure.

Additionally, when doing this process, Kicad encourages violation of the X2 spec: Filefunction,Other is intended to represent data _not_ representable by other Gerber X2 Filefunctions. When submitting Other as a technical layer, the generate Flefunction tag must be manually edited, or ignored by a fabricator.

However, with custom layers, this information is input directly into the Kicad PCB file. With certain fabs, you could simply submit the file directly, or generate the correct output without out-of-tool work.
For open-source projects, this also makes the project more consistent, as no communication is needed to explain that User1 is a technical layer.

Coming from the fab side of operations (as an employee of OSH Park), we routinely see the effect of requiring out-of-tool modifications to design files (mostly from other tools currently). Out of tool changes are very hard to standardize, and create a lot of frustration for users. The users most often hit by this are users of open-source projects, ordering a board without knowledge of abused layers and/or custom gerber sets and how they affect successful fabrication.

In the near future we plan on launching services that would leverage additional technical layers (Stiffener for flexible boards), which means that a great deal of projects will have to start abusing layers to generate a complete design. We're very loathe to make assumptions about the nature of the comment layers, and Kicad leaves no other way to properly present this data.