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.
The tests verifies that the `importadded` plugin updates added dates and
mtimes when importing:
* an album
* an album with file mtime preservation on
* singletons
* singletons with file mtime preservation on
It also verifies the the plugin doesn't update added dates when
re-importing:
* an album
* singletons
