Support background layers

[justvanrossum]

• Fontra source layers should be written as Glyphs layers
• Glyphs layers should be read as Fontra source layers
• Glyphs background layers should be mapped to Fontra source layers with a naming convention

In Fontra, background layers work with a naming convention, like this:

• The layer name should be named <parent-layer-name>^<background-layer-name>

Example:

• A GlyphSource's layerName attr is some-identifier-string
• A source bg layer is named background
• The fontra layer name would be some-identifier-string^background

In Fontra, this layer will be shown as a source layer background belonging to the source that uses some-identifier-string for layerName.

I think we should add an additional ^background to a Fontra layer name for a Glyphs background layer, possibly using a different separator char.

From Glyphs to Fontra: Fontra currently imports non-source layers as "inactive" sources. These should become source layers.