Implement StageManager, or something like it

In order to remove direct dependencies on Glass from the FXPluginToolkit, we need
a class that provides some basic management of the embedded primary stages.
This can be a private interface, perhaps on the desktop toolkit class.

At a minimum, this needs to provide:
    the ability to set the parent window for primary stages.
    setBackground (of the parent window), posibly other attributes)
     Once the window parent is set, primary stages should be constructed as child windows of this.
    Some means to control the Z-ordering of the Primary Stages (may require Glass changes)
    Also, the primary stage(s) should track the size of their parent.


Currently, a glass windows is constructed by the FX Toolkit and is used as the parent.
This works, but relies on platform specific behavior, and can not be extended to handle
the error dialog.