Misc fixes and cleanups

• Improve how the 'active' desktop is tracked (mostly useful for Wayland).
• Make it so xfdesktop --arrange sorts all desktops, but the context menu item only sorts the current desktop.
• Decouple the user action to manually cycle the backdrop from refreshing/reloading the whole desktop.
• Redesign the 'Desktop Icons' tab in the settings dialog at @lastonestanding's recommendation.
• Fix memory corruption due to a missing signal disconnect when destroying the icon view.