[REVIEW 3-5] performance improvement for fdo#48011

For https://bugs.freedesktop.org/show_bug.cgi?id=48011 an apparent
performance regression in writer, I'd like to propose
1856186951a70a0bcac4e0c3632ca4afe68c05e3 as a partial fix, code before
and after should be logical equivalent

i.e.
http://cgit.freedesktop.org/libreoffice/core/commit/?id=1856186951a70a0bcac4e0c3632ca4afe68c05e3
Window::GetChildCount and Window::GetChild are pretty slow as they
operate on the underlying linked lists. GetWindow lets you traverse the
linked lists directly. There are a few thousand windows to be traversed
on every keystroke. 

This helps a little, there's still some remaining underlying problem I
need to track down, but should improve things somewhat in the interim.

C.