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.
This wrapped string seems to be missing a space at the break.