[KBookMarksMenu] Set objectName for newBookmarkFolderAction

Summary:
This is needed if users of this class are expected to add the action
manually to their program actionCollection. Also add a method that
returns that action, just like the other three actions already have.

Test Plan: make && ctest

Reviewers: Frameworks, dfaure

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D28793