[tx] Add iFD to glyphs mappings to groups.plist

[kaydeearts]

Within groups.plist for CID-keyed UFO, add a dictionary of iFDs that point to a list of glyphs to identify which glyphs use which fdicts within the FDArray

[punchcutter]

On kd-fdarray-libplist branch I updated the test file tests/tx_data/input/cid_roundtrip/testCID.ufo with the new setup for FDArraySelect groups in groups.plist. Removed PrivateDicts from the FDArrays in lib.plist and moved the data in them to the top level. Also changed com.adobe.type.postscriptCIDMap to only map glyph names to CID instead of having iFD in there as well.

[kaydeearts]

Implemented in #1576 . Here's an example of what the groups.plist should look like!