Added support to Drag and Drop to file handlers, part of #111242.
If file handlers are registered with an import operator they can now be
invoked with drag and drop path data.
Import operators must either declare a `filepath` StringProperty or both
a `directory` StringProperty and a `files` CollectionProperty depending
on if they support single or multiple files respectively.
Multiple FileHandlers could be valid for handling a dropped path. When
this happens a menu is shown so the user can choose which exact handler
to use for the file.
Pull Request: https://projects.blender.org/blender/blender/pulls/116047
1254fee589