Add a slider UI widget

I'm not too happy with how I have to configure the step for each one
separately, but it's the best I can do at the moment.

internal/ui/driver.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"image"
 	"log"
+	"strconv"
 
 	"github.com/hajimehoshi/ebiten"
 	"github.com/hajimehoshi/ebiten/ebitenutil"
@@ -18,7 +19,6 @@ func init() {
 
 	// FIXME: these need further investigation / implementation
 	registerBuilder(menus.TypeOverlay, nil)
-	registerBuilder(menus.TypeSlider, nil)
 }
 
 const (
@@ -56,6 +56,7 @@ type Driver struct {
 	clickables []clickable
 	freezables []freezable
 	hoverables []hoverable
+	mouseables []mouseable
 	paintables []paintable
 	valueables []valueable
 
@@ -149,6 +150,36 @@ func (d *Driver) OnClick(id string, f func()) error {
 	return fmt.Errorf("Couldn't find clickable widget %q", id)
 }
 
+// FIXME: HURK. Surely I'm missing something? steps is value:offset
+func (d *Driver) ConfigureSlider(id string, steps map[int]int) error {
+	for _, clickable := range d.clickables {
+		if slider, ok := clickable.(*slider); ok && slider.id() == id {
+			slider.steps = steps
+			log.Printf("Found slider %#+v", slider)
+
+			return nil
+		}
+	}
+
+	return fmt.Errorf("Couldn't find slider %q", id)
+}
+
+func (d *Driver) ValueInt(id string, into *int) error {
+	var vStr string
+	if err := d.Value(id, &vStr); err != nil {
+		return err
+	}
+
+	value, err := strconv.Atoi(vStr)
+	if err != nil {
+		return err
+	}
+
+	*into = value
+
+	return nil
+}
+
 func (d *Driver) Update(screenX, screenY int) error {
 	// This will be updated while processing hovers
 	d.tooltip = ""
@@ -190,6 +221,10 @@ func (d *Driver) Update(screenX, screenY int) error {
 		d.mouseUpEvent(clickable, inBounds, mouseWasDown, mouseIsDown)
 	}
 
+	for _, mouseable := range d.mouseables {
+		mouseable.registerMousePosition(d.cursorOrig)
+	}
+
 	return nil
 }