Generalise Desktop.instance and Desktop.desktop for flexible window management

Desktop has static fields 'desktop' and 'instance'.

MyDesktopPane Desktop.desktop:
  - acts as the parent for dialogs (JvOptionPane methods and similar) (as a Component)
  - is the Component to which new frames are added, by Desktop.addInternalFrame()
  - provides MyDesktopPane.showMemoryUsage()

Desktop Desktop.instance
  - is tested for null (to check for headless operation)
  - provides getAllFrames()
  - is the target for method calls such as Desktop.instance.inputLocalFileMenuItem_actionPerformed()
  - is the ClipboardOwner
  - acts as a context provider (applet or desktop) for example to StructureSelectionManager.getStructureSelectionManager()
  - provides the Progress Bar
  - is the owner of a JDialog created by JalviewDialog.initDialogFrame() or WsJobParameters.showRunDialog()
  - is referenced for maximum dimensions of new panels

Would like to generalise and separate out these uses somehow
- to allow a different 'parent' to be injected for dialogs (generalise Desktop.desktop)
- to allow panels or internal frames to be added (generalise Desktop.instance and Desktop.addInternalFrame())
- specifically for a JalviewJS gui with a top level frame