This commit adds the experimental functionality in Contacts to import
VCard (*.vcf) files.

Since importing a contact means we have to take in untrusted/unvalidated
input, let's give a high-level view of what happens:

* Contacts starts a native file chooser dialog so the user can choose
  which file to import
* According to the chosen file, Contacts will launch a subprocess to do
  the actual parsing using a `Contacts.Io.Parser`. At this point, we
  only have a single subclass, which allows importing VCards.
* The helper process serializes the result to a `GLib.Variant`, and
  sends it to the main process, which will receive the result and
  parses it again.
* After the parsing operation is done, we can then start up a
  `ImportOperation`, which will import the contacts using libfolks' API.

Exporting contacts is quite a bit easier, since we don't have to deal
with untrusted input: we serialize the list of selected contacts and
asynchronously write each to the given output stream. In the app, that's
a user chosen file; in tests, that can be a string.

Fixes: #1
Fixes: #38