🐛 Fix export selected layer folder visibility issue

[huchenlei]

I found this bug when I am developing https://github.com/huchenlei/stable-diffusion-ps-pea.

Steps to reproduce:

• Set photopea in following state with a folder invisible, and select the folder or anything inside the folder.
  
• Click send to inpaint mask.
  
  You can see that the mask is not properly captured.

For comparison, if you select a top level layer and perform the same action:

The issue is caused by original code does not account LayerSet's visibility. For an ArtLayer to be visible on canvas, all it's parent's visibility all need to be true.

By default, add layer to add layer in current LayerSet if the activeLayer is a layerset or add a layer at the same level of the current active layer. So if we select the invisible folder layer, the new temp mask layer is added in the folder.

This PR port my implemenetation of exportSelectedLayerOnly from https://github.com/huchenlei/stable-diffusion-ps-pea, with extra feature of specifying export format and having a callback function to select which layer to export.

[huchenlei]

I am checking layer.selected instead for my usecase, as there can be multiple layers selected at the same time. I think for usecase in this repository checking layer.selected is equivalent to check app.activeDocument.activeLayer.