Fix context menu button for pages not showing up
fd8fec5b introduced a difference between "action" and "kirigamiAction"
properties. PrivateActionToolButton only checks kirigamiAction so use
that for the context button as well.
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
Differential Revision: https://phabricator.kde.org/D23499