[Request] Use user-defined layer names in Additional Layers box

[TheColorman]

When adding layers to the Additional Layers text box, the titles have to be the standard KiCad layers names. This makes sense for most layers, but custom user-defined layers (and sometimes even normal layers) usually have custom names.
E.g. in this example, the User.1 has been renamed F.glue in Board Setup, but you still need to search for User.1, and including F.glue does not work:

The only reason for this is because layer_info is indexed with 0 instead of 2 in these two sections of code:

Fabrication-Toolkit/plugins/utils.py

Lines 75 to 78 in a308115

	def get_layer_names(board, active_only=True):
	"""Returns a list of (active) layer names of the current board"""
	plotPlan = get_plot_plan(board, active_only)
	return [layer_info[0] for layer_info in plotPlan]

Fabrication-Toolkit/plugins/process.py

Line 77 in a308115

if self.board.IsLayerEnabled(layer_info[1]) or layer_info[0] in extra_layers:

I kept the KiCad layer names when adding the util in #152, but I think it would be more useful to be able to use the user-specified layer name directly. In theory changing these two lines should do it without affecting gerber generation.