When OwnedByDockingManagerWindow is set to false, windows disappear behind the main window when resize grouped gocument with GridSplitter. The issue related #349

Repro Steps:

1. In the TestApp, add to the style for the LayoutDocumentFloatingWindowControl or the LayoutAnchorableFloatingWindowControl.
2. Run the TestApp, and group 2 documents in float window.
3. Resize it with GridSplitter over another window and note that it goes behind the main window and is not visible.