improve checking of discrete spaces.

[LettError]

DSE reports incompatibilities between glyphs that are in different discrete locations.

[LettError]

Check group identities for each discrete location. Now it compares all groups across all discrete locations.

[typemytype]

the problems list should be a vanilla List2 where we can insert groups rows:

import vanilla

class Demo:

    def __init__(self):
        self.w = vanilla.Window((300, 150))
        items = [
            vanilla.List2GroupRow("Discrete space: width weight italic 0"),
            "A",
            "B",
            "C",
            "D",
            "E",
            "F",
            vanilla.List2GroupRow("Discrete space: width weight italic 1"),
            "1",
            "2",
            "3",
            "4",
            "5",
            "6",
            "7"
        ]
        self.w.list = vanilla.List2(
            "auto",
            items=items,
            allowsGroupRows=True,
            floatsGroupRows=True,
            allowsSorting=False
        )
        rules = [
            "H:|[list]|",
            "V:|[list]|"
        ]
        self.w.addAutoPosSizeRules(rules)
        self.w.open()
        

Demo()```