contact: Add QR code to share individual contacts

Sharing contacts in an easy and offline way is currently not possible. Most mobile phones have a camera and are capable of scanning QR codes. The vCard format is widely used to easily exchange contact information. A contact can be saved in vCard format into a QR code.

A button with a QR code icon is added next to the "favourite" and "edit" buttons. When the user presses this button, a dialog opens up, which shows a QR code containing the current contacts data in vCard format.

Closes #274 (closed)