Properly check enabled state of Action for ActionToolBar buttons

Summary:
The opacity binding overrides PrivateActionToolButton's enabled binding,
which means an Action's enabled property does not get checked for
ActionToolBar buttons.

Test Plan:
With kirigami2gallery, on the Misc Widgets page, change the enabled property
of one of the actions in the toolbar. Observe that the tool button now gets
disabled correctly.

Reviewers: Kirigami, ngraham, mart

Reviewed By: Kirigami, ngraham, mart

Subscribers: apol, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D24498