Changes the brightness of image using GammaAdjust QtGraphicalEffects

Summary:
Refs T6268

• uses grabToItem function to capture the modified image

Test Plan: there is just one problem the size of the image changes

Reviewers: Koko, mart

Reviewed By: Koko, mart

Subscribers: colomar

Maniphest Tasks: T6268

Differential Revision: https://phabricator.kde.org/D7468