Add support for KBookmarkOwner to communicate if it has tabs open

Summary:
Add support for KBookmarkOwner to communicate if it has tabs open. This
is used by KBookmarkMenu to only add the Bookmark Tabs as Folder...
entry if the KBookmarkOwner supports tabs and actually has tabs open.
KBookmarkMenu will also keep track of if the state of tabs open changes
and add/remove the menu entry accordingly.

Test Plan: Unit test added

Reviewers: Frameworks, ngraham, cfeck, dfaure

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D20209