Add code components 366-385; ScrollArea Follow, editor sync

A terminal with history, a Go-colored code block, a log viewer that
follows, a hex viewer, a memory map, a JSON tree, a line diff, a GAME
OVER crash screen, an FPS meter, profiler bars, breakpoints,
variables, command history, a shortcut list, a debug grid, a pixel
editor that exports string art, a palette editor, a sprite sheet, a
glyph map and a 6502 register view.

layout.ScrollArea takes Follow, and ChatLog now uses it. data.TreeNode
takes a Value drawn in its Slot. pixel gains Has, and Sprite.Art
writes MustSprite art. pixel.Editor takes the caller's value at each
input event, so a value changed between frames is never typed onto.

pixel/editor_test.go

@@ -336,3 +336,12 @@ func TestEditorSpaceDoesNotScroll(t *testing.T) {
 		t.Fatalf("value %q, scrolled to %v", value, scroll.Y)
 	}
 }
+
+func TestEditorTakesAValueChangedBetweenFrames(t *testing.T) {
+	h := newEdit(t, "help", 200, pixel.EditorOptions{})
+	h.focus()
+	// The caller clears it; a key lands before a frame.
+	h.value = ""
+	h.tt.Type("x")
+	h.want("x")
+}