Make it possible for actions to show submenus

Summary:
When using the ToolBarApplicationHeader we don't show the submenu, which
makes us miss some features from the ContextDrawer.
Still it doesn't seem to be possible to have submenus in QQC2.Menu, something
for the future I guess

Reviewers: Kirigami, mart

Reviewed By: Kirigami, mart

Subscribers: mart, ngraham, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D10613