Reactivate size slider in Red Eye Reduction tool

Summary:
While porting from KIntSpinBox to QSpinBox in 4efdfcdaef68 the
connection between the size slider and the spinbox got lost and
effectively disabled the slider.
This patch adds the signal/slot connections between both widgets
again and restores the functionality.

FIXED-IN: 18.08.0

Test Plan:

• Open Gwenview in View Mode
• Enter Red Eye Reduction tool
• Click in image to activate size option
• Check if slider and spinbox are synchronised

Reviewers: Gwenview, rkflx

Reviewed By: Gwenview, rkflx

Subscribers: rkflx

Differential Revision: https://phabricator.kde.org/D14329