Allow drop on advanced import dialog list

[Lymphatus]

This advanced import method cannot be like the 2.6 version where you can drag and drop images directly into the software. You must click on the option to add files or folders. You cannot drag and drop folders into the software, which is very inconvenient. It is more convenient to drag and drop into the software like in the 2.6 version.

Originally posted by @kofzhanganguo in #275 (comment)