Use set names for mark filtering sets

[behdad]

Right now the spec uses numbers, but has this weird part:
http://monotype.github.io/OpenType_Table_Source/otl_source.html#mark-glyph-sets-mark-filtering-sets

Begin set numbering at 0 (font tools should automatically renumber and report if you forget, but most important is that any references to mark glyph sets in GSUB or GPOS lookups must reference the renumbered sets.

My suggestion is to address sets by symbolic names (which might happen to be numbers), the same way lookup names and feature names are handled. Ie, the compiler will renumber references to these sets. Non-numeric set names will also be accepted. This makes creating meaningful sets much easer.

[behdad]

Thinking about it more, making set names symbolic makes it impossible to compile GSUB/GPOS without reading GDEF source first. So I'm going to retract my proposal. The wording is still weird. I suggest removing the "font tools should automatically renumber and report if you forget" part.