Fix issue with ambiguous overload for 'operator=' found as "src/relation...

Fix issue with ambiguous overload for 'operator=' found as "src/relationshipwidget.cpp:154:92: error: ambiguous overload for ‘operator=’ in ‘((RelationshipWidget*)this)->RelationshipWidget::tab_labels = {"", QTabWidget::tabText(int) const(1), QTabWidget::tabText(int) const(2), QTabWidget::tabText(int) const(3), QTabWidget::tabText(int) const(4)}’"

Please note that I am not skilled QT nor C++ developer and the fix works for linux, and should be confirmed on other targets like windows and mac.