Kirigami's ListSectionHeader has a default property that points to an embedded RowLayout's
data property, in an attempt (I guess?) to make it simpler to add things to it. However,
this does not work properly, at least not in combination with ActionToolBar. So instead,
override the section header's contentItem so we can specify the right layout properties.
See also https://bugs.kde.org/show_bug.cgi?id=415666
BUG: 415666