[KMoreTools] Reduce menu hierarchy

Summary: Skip 'More' submenu and attach its children directly to the menu when 'More' is the only submenu.

Test Plan:
Before:

After:

When 'More' isn't the only entry:

Reviewers: gregormi, dhaumann, ngraham

Reviewed By: dhaumann

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D13880