Add project components 421-435; Checkbox turns only when on

Fifteen project components: QuestBoard, TaskItem, TaskList, Milestones,
Burndown, Pomodoro, Priority, AssigneePicker, LabelPicker, TimeTracker,
Roadmap, SprintPanel, IssueCard, Checklist and WorkloadBar.

QuestBoard is one base that draws TODO, DOING and DONE; arrows move,
Alt with an arrow carries a card, and a drag carries it live. Priority
draws 12x12 bars, and Critical is on fire. Pomodoro banks time on PAUSE
and the tomato sleeps through the rest. Extreme inputs fail fast or
saturate instead of wrapping.

MyGo's CheckboxBase turns its value before Disabled is set, so
input.Checkbox now hands it a copy and keeps only turns of an enabled
base. The home scene test checks the sky row, not one pixel that
falling snow can cover.

project/priority.go

@@ -0,0 +1,78 @@
+package project
+
+import (
+	"fmt"
+
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/icon"
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type PriorityOptions struct {
+	// Scale is 1–4; zero means 1.
+	Scale int
+}
+
+// Bars rise with the level; # dots take its ink.
+var bars = [3]*pixel.Sprite{pixel.MustSprite(`
+		............
+		............
+		.........nnn
+		.........nnn
+		.........nnn
+		.....nnn.nnn
+		.....nnn.nnn
+		.....nnn.nnn
+		.###.nnn.nnn
+		.###.nnn.nnn
+		.###.nnn.nnn
+		.###.nnn.nnn`), pixel.MustSprite(`
+		............
+		............
+		.........nnn
+		.........nnn
+		.........nnn
+		.....###.nnn
+		.....###.nnn
+		.....###.nnn
+		.###.###.nnn
+		.###.###.nnn
+		.###.###.nnn
+		.###.###.nnn`), pixel.MustSprite(`
+		............
+		............
+		.........###
+		.........###
+		.........###
+		.....###.###
+		.....###.###
+		.....###.###
+		.###.###.###
+		.###.###.###
+		.###.###.###
+		.###.###.###`)}
+
+var levelInks = [...]pixel.Slot{pixel.Good, pixel.Warn, pixel.Bad, pixel.Bad}
+
+// Priority is a level's icon; critical is on fire.
+func Priority(c *ui.Context, l Level, o PriorityOptions) *ui.Element {
+	k := pixel.K(c)
+	say := l.word(k)
+	s := pixel.Or("Priority Scale", o.Scale, 1)
+	if s > 4 {
+		panic(fmt.Sprintf("project: Priority Scale %d is past 4", s))
+	}
+	return ui.Box(c).Size(k.D(12*s), k.D(12*s)).Role(ui.RoleImage).Label(say).Draw(func(p *ui.Painter, r ui.Rect) {
+		mark(k.Canvas(p, r), 0, 0, l, s)
+	})
+}
+
+// mark draws a level's icon at x, y.
+func mark(g pixel.Canvas, x, y int, l Level, s int) {
+	art := icon.Flame
+	if l < Critical {
+		art = bars[l]
+	}
+	g.Sprite(x, y, art, 0, levelInks[l], s)
+}