Propagate unused mouse press events in edit tools

Summary:
If one of the edit tools Crop or Red Eye Reduction is active,
zooming by Middle-Click or Ctrl+Left/Right-Click
does not work.
This patch ignores mouse press events not used by the edit tools
itself and thereupon they are propagated to the image view.

FIXED-IN: 18.08.0

Test Plan:

• Open Gwenview in View Mode
• Enter Crop or Red Eye Reduction tool
• Check if Ctrl+Left/Right-Click zoom works
• Check if (⇧) Middle-Click zoom works
• Check if crop tool ratio lock (Ctrl/⇧) works

Reviewers: Gwenview, rkflx

Reviewed By: Gwenview, rkflx

Subscribers: rkflx

Differential Revision: https://phabricator.kde.org/D14324