Implemented scene events and callback handlers for those events. Manager defaults also added.

examples/splashmenu/scenes/menuoption.go

@@ -0,0 +1,13 @@
+package splashmenu
+
+import (
+	"cosmos/diego/groovy"
+
+	"github.com/hajimehoshi/ebiten/v2"
+)
+
+type MenuOption struct {
+	Description    string
+	SelectionEvent groovy.SceneEvent
+	Mapping        ebiten.Key
+}