Set focus back to image view after leaving edit tool

Summary:
If you click on the toolbar of an image edit tool (crop, redeye)
the focus is lost after leaving the tool and you cannot switch images
with cursor keys.
This patch resets the focus to image view after leaving the edit tool.

BUG: 321091

Test Plan:
* Open image in Gwenview View Mode (normal or fullscreen)
* Start crop tool ({key Shift C})
* Click on {nav Crop} or {nav Cancel}
* Switching images with cursor keys should work as usual

Reviewers: #gwenview, rkflx

Reviewed By: #gwenview, rkflx

Subscribers: rkflx, huoni

Differential Revision: https://phabricator.kde.org/D11899