Fix focus chain

Prior to this commit pressing Tab repeatedly would bring the focus to the end of the status bar but not further.

This commit makes sure the tab focus doesn't get stuck on the invisible tab bar by explicitly removing the DolphinTabBar from the focus chain while it is hidden. I don't understand why pressing Tab doesn't do anything for the invisible tab bar, but removing an invisible and currently useless widget from the focus chain seems sensible in any case.

Improve the accessibility autotest to prevent regressions concerning this.