The Application Menu Bar has been backported from Plasma 5.9.5 to Leap 42.3 Plasma 5.8.6 LTS
The "Application Menu Bar" was been backported from Plasma 5.9.5 and is now available in the latest build of 42.3.
Just right click on the desktop and select the Application Menu Bar. Place the bar at the top pf the screen.
The menus i.e kate will appear on the menu bar (from most Qt apps) excluding Firefox, Chrome, Chromium and GNOME applications.
Then select Configure Desktop->Application Style->Fine Tuning - click on the Application dropdown arrow and
select the "Application Menu Bar" widget.