Desktop icon rework

[jackpot51]

Makes it possible to manually position icons across displays, fixing #569 and #1239. Draft until implementation is complete.

• Per display desktop icons
• Automatically determine primary display
• Manual preference for primary display
• Manual positioning of desktop files
• Manual positioning of mounted drives and trash
• Correct interaction between sort and manually positioned items
• Adjust desktop icons when displays change
• Cleanup

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