Add support for re-importing added dates.

When re-importing, `item.added` and `album.added` will be preserved if
their path match what they are replacing.

Please note that it isn't ready to be merged yet.

@geigerzaehler here is the branch we discussed in #925.

As I mentioned in #925 the reimport in this branch is based on matching paths of pre re-import items and albums and post re-import items and albums. It will not work if the paths are changed during the reimport.

I plan to revert the changes to `db.py` and to move most of the reimport logic into a new method `reimport_metadata` in the importer. That will sacrifice performance for clarity.
We might be able to simplify this substantially if we were allowed to add the album first and *then* call `remove_replaced`. Then we wouldn't need to keep around `replaced_albums` since we'd have the `task.album` object available when doing the matching. I *think* that should be possible…