DesktopIntegration: Improved icon/shortcut handling

--HG--
rename : src/DesktopIntegration/Unix/DesktopManager.cs => src/DesktopIntegration/Unix/FreeDesktop.cs
rename : src/DesktopIntegration/Windows/ShortcutManager.cs => src/DesktopIntegration/Windows/Shortcut.cs