Implement the options menu, part 1

This commit implements loading and saving options from/to config, and
enough UI toolkit magic to allow changes to boolean options to be
persisted.

We now respect the "play movies" setting and take screen resolution
from the config file.

internal/ui/widget.go

@@ -15,6 +15,7 @@ type Widget struct {
 	// Position on the screen in original (i.e., unscaled) coordinates
 	Bounds  image.Rectangle
 	Tooltip string
+	Value   string // #dealwithit for bools and ints and so on :p
 
 	OnHoverEnter func()
 	OnHoverLeave func()
@@ -40,6 +41,8 @@ type Widget struct {
 	path   []int
 	record *menus.Record
 	sprite *assetstore.Sprite
+
+	valueToImage func() *ebiten.Image
 }
 
 func (w *Widget) Disable() {
@@ -86,7 +89,7 @@ func (w *Widget) Image(aniStep int) (*ebiten.Image, error) {
 		return w.disabledImage, nil
 	}
 
-	if w.hoverState && w.mouseButtonState {
+	if w.mouseButtonDownImage != nil && w.hoverState && w.mouseButtonState {
 		return w.mouseButtonDownImage, nil
 	}
 
@@ -94,5 +97,9 @@ func (w *Widget) Image(aniStep int) (*ebiten.Image, error) {
 		return w.hoverAnimation[(aniStep)%len(w.hoverAnimation)], nil
 	}
 
+	if w.valueToImage != nil {
+		return w.valueToImage(), nil
+	}
+
 	return w.sprite.Image, nil
 }