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.

input/disabled_test.go

@@ -169,3 +169,22 @@ func TestDisabledWhileFocusedTakesNoInput(t *testing.T) {
 		})
 	}
 }
+
+func TestInertSteppersTakeNoPress(t *testing.T) {
+	n, v := 5, 5.0
+	tt := ui.NewTester(func(c *ui.Context) {
+		pixel.Use(c, pixel.Settings{Palette: pixel.SpringDay, Inert: true})
+		ui.Row(c).Children(func() {
+			input.Stepper(c, "Lives", &n, input.StepperOptions{Max: 9})
+			input.NumberInput(c, "Speed", &v, input.NumberInputOptions{Max: 10})
+		})
+	}, 600, 100)
+	s, f := find(t, tt, "Lives"), find(t, tt, "Speed")
+	// Arrows sit left of a stepper, right of a field.
+	tt.Press(s.X-12, s.Y+8)
+	tt.Release(s.X-12, s.Y+8)
+	tt.ClickAt(f.X+f.W+10, f.Y+4)
+	if n != 5 || v != 5 {
+		t.Fatalf("inert arrows set %d and %v", n, v)
+	}
+}

input/numberinput.go

@@ -86,8 +86,9 @@ func NumberInput(c *ui.Context, label string, value *float64, o NumberInputOptio
 
 // arrowBox is a half-height step button the pointer presses.
 func arrowBox(c *ui.Context, k pixel.Kit, sp *pixel.Sprite, disabled bool) bool {
+	disabled = disabled || pixel.Current(c).Inert
 	b := ui.Box(c).Size(k.D(10), k.D(pixel.LineHeight/2)).Role(ui.RoleNone)
-	down := b.Pressed()
+	down := b.Pressed() && !disabled
 	b.Draw(func(p *ui.Painter, r ui.Rect) {
 		g := k.Canvas(p, r)
 		dy := 0

input/stepper.go

@@ -73,7 +73,7 @@ type hold struct {
 // held counts an arrow's steps: one, then ten a second.
 func held(c *ui.Context, k pixel.Kit, sp *pixel.Sprite, disabled bool) int {
 	b := ui.Box(c).Size(k.D(12), k.D(pixel.LineHeight+4)).Role(ui.RoleNone)
-	down := b.Pressed() && !disabled
+	down := b.Pressed() && !disabled && !pixel.Current(c).Inert
 	h := ui.Local(b, holdKey{}, func() hold { return hold{} })
 	b.Draw(func(p *ui.Painter, r ui.Rect) {
 		g := k.Canvas(p, r)