[PlasmaComponents3] Fix checkable toolbutton background
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.
Checkable PC3 ToolButtons now look checked when checked
No change for non-checkable PC3 ToolButtons
Reviewers: Plasma, mart
Reviewed By: Plasma, mart
Differential Revision: https://phabricator.kde.org/D24402