ImportAdded support for in-place and link imports

Fix for the `ImportAdded` plugin crashing during in-place imports (#1107).

Add support for the new link imports introduced in Beets 1.3.9.

Note that link-imports that preserve file modification times will follow
the links and preserve the mtimes on the link targets. The mtimes on the
links aren't modified.
Add a missing space
Merge pull request #1170 from silb/importadded-in-place-link-support

ImportAdded support for in-place and link imports
Changelog for #1170