Port away from HeaderFooterToolbar

When I created this component, I didn't know that the QQC2 `ToolBar`
component can do all the same things and even has built-in support for
being displayed as a header or a footer. So let's use that instead.
1 job for !170 with port-away-from-HeaderFooterToolbar in 8 minutes and 51 seconds (queued for 2 seconds)
latest detached
Status Job ID Name Coverage
  Build
passed #68474
linux kf5-qt5 qt5.12

00:08:51

86.0%