Toggle between background and node layer in the edit view.

Sometimes you want to place a background image or text, but it is hard to click these background nodes. Therefore add a menu item and a shortcut. For switch between these layers.

if the node layer is selected. everything should work like now.

if the background layer is selected. the nodes should move behind the background layer, so that they are not hindering the editing of the background layer.

首先定位编辑视图的图层选择行为、菜单项注册和快捷键处理。跟踪背景节点和普通节点的选择与渲染方式，然后验证切换图层时是否保留节点图层的当前行为，并允许在不遮挡节点的情况下编辑背景内容。