ImplGetSubChildWindow has special code to handle a TabControl and
its pages. If the function finds a TabControl as a child it'll
return its window and then recurse into the current page.
This currently breaks in the case where the initial parent is a
TabControl. where the function will walk all of the tab pages.
The function also ignores border windows, but not if the initial
parent is a border window.
This patch correctly handles both cases and as a bonus drops all
the special page handling during child iteration.
Add warnings for different visibility settings in the "label-for"
and "labelled-by" elements.
Sine LO doesn't support multiple annotations per accessibility
element, we have to switch them manually, if we change the
visibility of one element of a pair.
