Expose ActionToolBar's overflow button icon name

Summary:
This exposes the ActionToolBar overflow button's icon name as a property on
ActionToolBar. This allows changing the icon, primarily to using a hamburger
icon for indicating a "global menu".

Test Plan: Overflow menus still look the same.

Reviewers: Kirigami, mart

Reviewed By: Kirigami, mart

Subscribers: plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D25943