Allow auto-creation of tiles from bulk images

[amuck-gorilla]

In order to create a TileSet, the Godot editor offers the ability to drag and drop images onto the atlas list one by one or in bulk. The editor also offers a feature to automatically create tiles from an image, based on transparency.

However, if more than one image is dropped into the workspace, the option to automatically create tiles does not appear, requiring users to either manually create tiles for all of their images, or drag and drop images one by one to create tiles automatically.

Therefore I propose this should be fixed, with the option to automatically create tiles from images in bulk.