Additional layers: can't select user-defined layers

[TheColorman]

When exporting using Fabrication Toolkit, we have the ability to write the names of additional layers to be exported, but this doesn't include any user defined layers, e.g. User.[0-9]. It seems the layers you can use are hardcoded here: https://github.com/bennymeg/Fabrication-Toolkit/blob/ab0b336ed9207d7636f2f6a2a7d2d4847c0c3b5a/plugins/config.py#L15-L56

I suggest using a way to dynamically get the enabled layers, such as

layers = []
i = pcbnew.PCBNEW_LAYER_ID_START
while i < pcbnew.PCBNEW_LAYER_ID_START + pcbnew.PCB_LAYER_ID_COUNT:
    layer_std_name = pcbnew.BOARD.GetStandardLayerName(i)
    layer_name = pcbnew.BOARD.GetLayerName(board, i)

    layers.append((layer_std_name, i, layer_name))
    i += 1

Adapted from Board2Pdf.

Additionally, the additional layers input box seems to support a list of comma-separated layer names. https://github.com/bennymeg/Fabrication-Toolkit/blob/ab0b336ed9207d7636f2f6a2a7d2d4847c0c3b5a/plugins/process.py#L72 This isn't very clear to the user, especially as the autocomplete doesn't acknowledge attempts at adding more than one layer, so I feel like some type hint would be useful.

I'm working on a PR to address these issues, but I'd like to hear some feedback about it here first.

[bennymeg]

Sounds good. Regarding the autocomplete, I agree, I could not make it work properly for more than one layer, and the UI is quite limited and could not make it look nice, so I opted to ignore it. If you could add it properly, I'm all for it.

[TheColorman]

I've opened PR #152 that makes the autocomplete list dynamically generated. I tried to play around with having a list of layers underneath the text box that you could add/remove from, but couldn't get anything that looked nice, so for now I've just updated the textbox hint.

[bennymeg]

Thank you!