Replace some obsolete methods with non-obsolete ones

Summary:
There are some deprecated methods in the code:

https://build.kde.org/job/Applications/job/gwenview/job/kf5-qt5%20FreeBSDQt5.13/4/warnings12Result/category.2119183113/

Test Plan:
1. Run Gwenview, select an image.
2. Use Edit -> Crop to ensure that cropping is working as expected.
3. Press F11 to go to the full screen mode. The top panel should appear as expected.

Reviewers: #gwenview, ngraham

Reviewed By: #gwenview, ngraham

Subscribers: meven, ngraham, tommo

Tags: #gwenview

Differential Revision: https://phabricator.kde.org/D22527