We don't yet have a real QML tab widget (https://bugs.kde.org/show_bug.cgi?id=394296)
so we have to manufacture our own by combining a TabBar with a frame in which the
content sits.

This patch slightly improves the look and feel of our manufactured tab widget
by moving the tab portion down a bit so it overlaps the frame, which does a
better job of mimiching the QWidgets version. It's not perfect, but it's at
least no worse with all the themes I tried, and for some it looks better.

Breeze, before:

Breeze after:

Oxygen, before:

Oxygen, after:

MSWindows, before:

MSWindows, after:

Fusion, before:

Fusion, after:

QtCurve, before:

QtCurve: after:

It's not always perfect, but for some it's better IMO, and it's at least not worse for any.