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/digitalclock.go

@@ -0,0 +1,43 @@
+package datetime
+
+import (
+	"fmt"
+
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/text"
+)
+
+type DigitalClockOptions struct {
+	// Scale is 1–4; zero means 2.
+	Scale int
+}
+
+// DigitalClock shows HH:MM:SS in seven segments; colons blink.
+func DigitalClock(c *ui.Context, o DigitalClockOptions) *ui.Element {
+	k := pixel.K(c)
+	sc := pixel.Or("DigitalClock Scale", o.Scale, 2)
+	if sc > 4 {
+		panic(fmt.Sprintf("datetime: DigitalClock Scale %d outside 1–4", sc))
+	}
+	k.Tick(c, 1)
+	now := k.Now()
+	colon := func() {
+		ui.Box(c).Size(k.D(3*sc), k.D(11*sc)).Role(ui.RoleNone).Draw(func(p *ui.Painter, r ui.Rect) {
+			g := k.Canvas(p, r)
+			// The colon blinks each half second, unless reduced.
+			if k.Reduce() || g.Step(2)%2 == 0 {
+				g.Rect(sc, 3*sc, sc, sc, pixel.Bad)
+				g.Rect(sc, 7*sc, sc, sc, pixel.Bad)
+			}
+		})
+	}
+	return ui.Row(c).Gap(k.D(2 * sc)).Role(ui.RoleText).Label(now.Format("15:04:05")).Children(func() {
+		text.SevenSegment(c, now.Format("15"), text.SevenSegmentOptions{Scale: sc})
+		colon()
+		text.SevenSegment(c, now.Format("04"), text.SevenSegmentOptions{Scale: sc})
+		colon()
+		text.SevenSegment(c, now.Format("05"), text.SevenSegmentOptions{Scale: sc})
+	})
+}