Add a page to display ActionToolBar and some of its features

Summary:
ActionToolBar has gained quite a lot of functionality. So add a page
to highlight some of it.

There's a screencapture of the page in D25940

Test Plan: The new page displays properly.

Reviewers: #kirigami, mart

Reviewed By: #kirigami, mart

Subscribers: plasma-devel

Tags: #plasma, #kirigami

Differential Revision: https://phabricator.kde.org/D25941