fdo#70465 SvTreeListBox::GetLevelChildCount refactored

Instead of iterating siblings returning count of parents child vector.
This slightly improves loading performance of "Expert Config" when accessibility enabled.

Change-Id: Ide1af3df19efaae9c0cc92086456bf3520ee5dd2
Reviewed-on: https://gerrit.libreoffice.org/7106
Reviewed-by: Kohei Yoshida <libreoffice@kohei.us>
Tested-by: Kohei Yoshida <libreoffice@kohei.us>