[PlasmaComponents3] Fix checkable toolbutton background

Summary:
This fixed the background of a checkable PC3 ToolButton not being visible when checked.
The PC3 Button and PC2 ToolButton did not suffer from the issue, but the PC3 one did.

Test Plan:
Checkable PC3 ToolButtons now look checked when checked

No change for non-checkable PC3 ToolButtons

Reviewers: Plasma, mart

Reviewed By: Plasma, mart

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D24402