Add a displayHint property to Action
This property can be used to indicate preferences on how to display the
action to implementations using Action. It mimics some of the behaviour
of QAction's priority property, but is more explicit and allows some
behaviour the priority property would not.
Additionally, this changes PrivateActionToolButton to make use of the
displayHint property for its showText and showMenuArrow properties.
Tool buttons created through actions in Kirigami gallery still show
their text and menu arrows.
Reviewers: Kirigami, mart
Reviewed By: Kirigami, mart
Subscribers: broulik, leinir, plasma-devel
Differential Revision: https://phabricator.kde.org/D25176