Make it possible to modify contacts

Summary:
Includes a vcard field so that backends can feed all of the data through
it.
Introduces the AbstractEditableContact that when subclassed it will be
able to edit contacts by overriding setCustomProperty.
The vcard was introduced because I'd expect contact applications using
the vcard data as an interface rather than implementing every property
one by one.

Test Plan: Modified my contacts several times using plasma-phonebook.

Reviewers: #frameworks, jbbgameich, bshah

Reviewed By: jbbgameich

Subscribers: kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D22968