klipper: highlight the first item in popup menu

Reason for the change

In the old Meta+V QtWidgets menu popup, the top item was always highlighted, so you could search until the item you wanted was on top, and then immediately hit the return key.

With the new QML-based one, you have to hit the down arrow before the return key. It's a tiny bit slower and more annoying.

This restores the old behavior.

Test plan

Screenshots or screen recordings

Bugs fixed

BUG: 491142 FIXED-IN: 6.2