Add datetime components 196-215; Kit.At; inert stepper arrows

Calendars, date and time inputs, pickers, clocks, timers, a sun dial,
a season wheel, an agenda and a day schedule. Kit.At rebuilds at a
moment (relative times, midnight). Stepper and NumberInput arrows stop
under Inert. DateInput parks a popup focus as its own.

datetime/analogclock.go

@@ -0,0 +1,45 @@
+package datetime
+
+import (
+	"math"
+
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type AnalogClockOptions struct {
+	// Size is the face's width in dots; zero means 48.
+	Size int
+}
+
+// AnalogClock ticks each second; its hands are Bresenham lines.
+func AnalogClock(c *ui.Context, o AnalogClockOptions) *ui.Element {
+	k := pixel.K(c)
+	size := pixel.Or("AnalogClock Size", o.Size, 48)
+	k.Tick(c, 1)
+	now := k.Now()
+	h, m, s := now.Clock()
+	// hand ends at turn (0–1 from twelve) and length r.
+	hand := func(cx, cy int, turn float64, r int) (int, int) {
+		a := turn * 2 * math.Pi
+		return cx + int(math.Round(float64(r)*math.Sin(a))), cy - int(math.Round(float64(r)*math.Cos(a)))
+	}
+	return ui.Box(c).Size(k.D(size), k.D(size)).Role(ui.RoleImage).Label(now.Format("15:04:05")).Draw(func(p *ui.Painter, r ui.Rect) {
+		g := k.Canvas(p, r)
+		cx, cy, rad := size/2, size/2, size/2-1
+		g.Disc(cx, cy, rad, pixel.Face)
+		g.Circle(cx, cy, rad, pixel.Edge)
+		for i := range 12 {
+			x, y := hand(cx, cy, float64(i)/12, rad-3)
+			g.Dot(x, y, pixel.Edge)
+		}
+		x, y := hand(cx, cy, (float64(h%12)+float64(m)/60)/12, rad/2)
+		g.Line(cx, cy, x, y, pixel.Ink)
+		x, y = hand(cx, cy, (float64(m)+float64(s)/60)/60, rad-5)
+		g.Line(cx, cy, x, y, pixel.Ink)
+		x, y = hand(cx, cy, float64(s)/60, rad-4)
+		g.Line(cx, cy, x, y, pixel.Accent)
+		g.Dot(cx, cy, pixel.Accent)
+	})
+}