Rework the UI framework

Interface is now Driver, and Widget is now a set of interfaces with a
struct per widget type. This should make it easier to add other types.

internal/ui/animation.go

@@ -4,12 +4,16 @@ import (
 	"github.com/hajimehoshi/ebiten"
 )
 
+var (
+	SpeedDivisor = 2
+)
+
 type animation []*ebiten.Image
 
-func (a animation) image(step int) *ebiten.Image {
+func (a animation) image(tick int) *ebiten.Image {
 	if len(a) == 0 {
 		return nil
 	}
 
-	return a[step%len(a)]
+	return a[(tick/SpeedDivisor)%len(a)]
 }