Rename Glyphs custom parameter handler doesn't affect kerning

[simoncozens]

I don't need this in glyphsLib but I discovered in when working on babelfont: The Glyphs "Rename Glyphs" custom parameter also swaps the names of glyphs in the kerning tables as well; we just swap the outlines, meaning the kerning (or lack of kerning) of the old glyph is applied to the new glyph.

[schriftgestalt]

Glyphs handles this in two ways.

1. the kerning uses identifiers instead of the glyph name as keys in the kerning dict. That allows renaming the glyphs without the need to update the kerning dict all the time. The identifiers are swapped in when loading the file and replaced by names when saving.
2. The Rename Glyphs parameter actually replaces the names, not the outlines. And because of the identifiers, the kerning sticks to the glyph even with a new name.