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/7106Reviewed-by: Kohei Yoshida <libreoffice@kohei.us>
Tested-by: Kohei Yoshida <libreoffice@kohei.us>

svtools/source/contnr/treelistbox.cxx

@@ -937,15 +937,16 @@ sal_uLong SvTreeListBox::GetLevelChildCount( SvTreeListEntry* _pParent ) const
 {
     DBG_CHKTHIS(SvTreeListBox,0);
 
-    sal_uLong nCount = 0;
+    //if _pParent is 0, then pEntry is the first child of the root.
     SvTreeListEntry* pEntry = FirstChild( _pParent );
-    while ( pEntry )
-    {
-        ++nCount;
-        pEntry = NextSibling( pEntry );
-    }
 
-    return nCount;
+    if( !pEntry )//there is only root, root don't have children
+        return 0;
+
+    if( !_pParent )//root and children of root
+        return pEntry->pParent->maChildren.size();
+
+    return _pParent->maChildren.size();
 }
 
 SvViewDataEntry* SvTreeListBox::GetViewDataEntry( SvTreeListEntry* pEntry ) const