Let pcbnew check for layer names

[Drinausaur]

The current kiplot code checks for layer names.
But it is hardcoded, and in Kicad, we can change inner layer names to whatever we want.

This patch relies on pcbnew API to check if the layer is correct or not.

[set-soft]

Hi @Drinausaur !
Take a look at my fork: https://github.com/INTI-CMNB/kiplot
It supports the KiCad names and the legacy names used in the original code.