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.

cmd/view-menu/main.go

@@ -35,12 +35,12 @@ func main() {
 		log.Fatalf("Couldn't load menu %s: %v", *menuName, err)
 	}
 
-	iface, err := ui.NewInterface(menu)
+	driver, err := ui.NewDriver(menu)
 	if err != nil {
 		log.Fatalf("Couldn't initialize interface: %v", err)
 	}
 
-	win, err := ui.NewWindow(iface, "View Menu: "+*menuName, *winX, *winY)
+	win, err := ui.NewWindow(driver, "View Menu: "+*menuName, *winX, *winY)
 	if err != nil {
 		log.Fatal("Couldn't create window: %v", err)
 	}