Dragging window appears behind main window

[mpondo]

When OwnedByDockingManagerWindow is set to false, floating windows disappear behind the main window when being dragged.

Repro Steps:

1. In the TestApp, add <Setter Property="OwnedByDockingManagerWindow" Value="False" /> to the style for the LayoutDocumentFloatingWindowControl or the LayoutAnchorableFloatingWindowControl.
2. Run the TestApp, and float one of the windows.
3. Drag it over another window and note that it goes behind the main window and is not visible.

Additional Information

• The OwnedByDockingManagerWindow was added in Support minimizing floating windows independently of main window #276 to support minimizing and maximizing floating windows separately from the main window.
• A more complete fix to per-monitor DPI issues #326 changed the ownership of the OverlayWindow. When the floating window is not owned, it is not allowed to be inserted between the OverlayWindow and its owner and is forced behind.