Refactor conversation stuff

cutscene/conversation/ConversationAction.gd

@@ -0,0 +1,12 @@
+class_name ConversationAction
+
+static func textboxCallable(params:Dictionary) -> int:
+  assert(params.has('label'))
+  await UI.TEXTBOX.setTextAndWait(params['label'])
+  return Cutscene.CUTSCENE_CONTINUE
+
+static func getTextboxCallable(label:String) -> Dictionary:
+  return {
+    "function": textboxCallable,
+    "label": label
+  }