Fix context menu button for pages not showing up

Summary:
fd8fec5b introduced a difference between "action" and "kirigamiAction"
properties. PrivateActionToolButton only checks kirigamiAction so use
that for the context button as well.

Test Plan:
Start kirigami gallery, go to the "buttons" page, see that it now has a
context menu again.

Reviewers: Kirigami, mart, ngraham

Reviewed By: Kirigami, ngraham

Subscribers: plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D23499