Fix background color for image view toolbar (crop/red-eye)

Summary:
The slide-in toolbar in image view, used by the crop and red-eye tools, sets its content dynamically.
This content wasn't getting the app's palette, resulting in the background colour being fixed.
This was only a serious problem in light color schemes in fullscreen mode, where the background color of the tool stayed light but the text also went light (due to fullscreen being dark themed).

This patch simply manually sets the toolbar's content's palette when it changes.

Normal, before:
{F5726450}

Normal, after:
{F5727191}

Fullscreen, before
{F5726451}

Fullscreen, after
{F5727192}

Test Plan:
First set your color scheme to something light (e.g. Honeycomb, Zion).
Go to {nav View}, press `F4`, click the {nav Operations} tab, click {nav Crop} or {nav Red Eye Reduction}.
Go to fullscreen. The toolbar should be dark.

Reviewers: #gwenview, rkflx, ngraham

Reviewed By: #gwenview, rkflx, ngraham

Subscribers: ngraham

Differential Revision: https://phabricator.kde.org/D10763