Slicer3:Extensions:TODO-2009-06-16

• search vtkSlicerApplication::ModulePaths when consructing the installed extension listings
• warn user if cache path is not writable
• check that ModulePaths disable/enable is not inadvertantly set
• what happens when the Desktop is used as the "module cache path"
• add vtkSlicerApplication::ModulePaths if found downloaded extensions are not there