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