Add data components 151-175; clear MyGo row highlights

Lists, tables, trees, grids, inventories and records in data/.
Outline and GridView build on ui.List: ui.Outline strokes its arrows
and ui.GridView spaces and tints items. The pixel theme keeps MyGo's
row highlights clear so scrolled rows stay on whole dots; off lists
take a veil against the wheel. Adds Canvas.Silhouette, SwitchOptions
Label and exports Rarity.Slot.

data/data.go

@@ -0,0 +1,89 @@
+// Package data shows collections: lists, tables, trees, grids.
+package data
+
+import (
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+var (
+	pointer = pixel.MustSprite(`
+		#..
+		##.
+		###
+		##.
+		#..
+	`)
+	up = pixel.MustSprite(`
+		..#..
+		.###.
+		#####
+	`)
+	down = pixel.MustSprite(`
+		#####
+		.###.
+		..#..
+	`)
+)
+
+// well sinks box and finishes it as st says.
+func well(c *ui.Context, box *ui.Element, st pixel.State) {
+	k := pixel.K(c)
+	box.Padding(k.D(2)).Draw(func(p *ui.Painter, r ui.Rect) {
+		g := k.Canvas(p, r)
+		g.Frame(0, 0, g.W, g.H, pixel.Sunken, pixel.Well)
+	}).DrawOver(func(p *ui.Painter, r ui.Rect) {
+		k.Canvas(p, r).Finish(st)
+	})
+}
+
+// veil covers an off list; the wheel stops on it.
+func veil(c *ui.Context, off bool) {
+	if off {
+		ui.Box(c).Absolute().Top(0).Left(0).Fill()
+	}
+}
+
+// row is a shown tree row, with its parent.
+type row[K comparable] struct {
+	key    K
+	depth  int
+	branch bool
+	parent int
+}
+
+// flatten lists the rows shown under roots, inside open branches.
+func flatten[K comparable](roots []K, children func(K) []K, opened map[K]bool) []row[K] {
+	var out []row[K]
+	var walk func(items []K, depth, parent int)
+	walk = func(items []K, depth, parent int) {
+		for _, it := range items {
+			kids := children(it)
+			out = append(out, row[K]{it, depth, kids != nil, parent})
+			if opened[it] && kids != nil {
+				walk(kids, depth+1, len(out)-1)
+			}
+		}
+	}
+	walk(roots, 0, -1)
+	return out
+}
+
+// steer: Right opens or descends; Left closes or climbs.
+func steer[K comparable](rows []row[K], at int, opened map[K]bool, right bool) (to int, toggled bool) {
+	r := rows[at]
+	switch {
+	case right && r.branch && !opened[r.key]:
+		opened[r.key] = true
+		return at, true
+	case right && r.branch && at+1 < len(rows):
+		return at + 1, false
+	case !right && r.branch && opened[r.key]:
+		delete(opened, r.key)
+		return at, true
+	case !right && r.parent >= 0:
+		return r.parent, false
+	}
+	return at, false
+}