SplitLayout: Fix #12261.

- SetItemCollapsed()/IsItemCollapsed() didn't properly handle inverting
  values when mapping between the collapsed flag and the layout item's
  internal visibility flag, leading to them having the opposite semantics
  from what their name would suggest.

src/kits/interface/SplitLayout.cpp

@@ -1,5 +1,6 @@
 /*
  * Copyright 2006-2009, Ingo Weinhold <ingo_weinhold@gmx.de>.
+ * Copyright 2015, Rene Gollent, rene@gollent.com.
  * All rights reserved. Distributed under the terms of the MIT License.
  */
 
@@ -493,14 +494,14 @@ BSplitLayout::SetCollapsible(int32 first, int32 last, bool collapsible)
 bool
 BSplitLayout::IsItemCollapsed(int32 index) const
 {
-	return _ItemLayoutInfo(ItemAt(index))->isVisible;
+	return !_ItemLayoutInfo(ItemAt(index))->isVisible;
 }
 
 
 void
 BSplitLayout::SetItemCollapsed(int32 index, bool collapsed)
 {
-	ItemAt(index)->SetVisible(collapsed);
+	ItemAt(index)->SetVisible(!collapsed);
 
 	InvalidateLayout(true);
 }