kcms/autostart: Proper popup menu behaviour and removed workaround hack
Fixes the first-time menu opening being misplaced with the workaround. This is done by changing menu.popup() to menu.open() and setting the menu location with y: -height - Kirigami.Units.smallSpacing. Just y: -height also works, but it is done this way on plasma-pa, so I followed suit.
Behaviour before:
Behaviour after: