Let the application control the menu further

Summary:
Introduces a property so that triggering leaf categories doesn't change the
state of the menu.
Introduces a function so the application can reset the menu to its initial state

Reviewers: Kirigami, mart

Reviewed By: mart

Subscribers: colomar, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D2652