This commit changes a couple of things about the import UI:

First of all, rather than showing a "simple" message dialog, we have a
custom `ImportDialog` class to allow more extended interfaces.

Since we now have this new-fangled dialog, we can actually use it to
show the list of contacts that would be imported, so we can give a clear
idea to the user what we're exactly importing.

Finally, thanks to the custom UI, this allows us to enable importing
from multiple files, rather than just a single file.

Fixes: #328
Related: #329