Make it possible to use KContacts without Qt::Gui

Summary:
Only QImage in KContacts::Picture needs Qt::Gui.
If Pictures isn't used in consumer, we do not need to threat them to
link Qt:Gui for nothing.

Test Plan:
 Built all users of KContacts:
  * messagelib
  * akonadi-contacts
  * akonadi-import-wizard
  * akonadi-search
  * kdepim-addons
  * korganizer
  * kdepim-runtime (see fix: https://commits.kde.org/kdepim-runtime/bccc569b2c049e6eb1c43b45ba0e25bb39868913)

Reviewers: mlaurent

Subscribers: #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D4286