allow to add application actions on an open menu

Summary:
even if the menu has already been created, cause setapplicationActions
to add actions in the drop menu.

Test Plan: twsted on plasma drop menu, menu opens and in a second time gets the extra actions

Reviewers: Plasma, dfaure

Reviewed By: dfaure

Subscribers: dfaure, hein, plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D4620