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.

Change-Id: Ie8699dae8d08628b66b33e0704237b9e219874bc
Reviewed-on: https://gerrit.libreoffice.org/61037
Tested-by: Jenkins
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>