Reduce contents of drop menu when dropping abstract items
When the user drags an item from Kickoff/Task Manager/etc. that's internally implemented as a .desktop file (e.g. an application), this implementation detail leaks out when the menu shows "Move" or "Copy" in it. And using the "Move" action can break the item's integration by removing its .desktop file from the normal location!
We want to prevent this, but also allow the user to continue moving or copying .desktop files normally via their file managers.
To achieve both objectives, don't show Move and Copy options when the item being dragged is accompanied by a special MIME hint indicating that it's an abstract representation of something.
This has no effect on its own; clients need to opt into the behavior.
Please review carefully as I'm like the dog who has no idea what he's doing here. This felt conceptually sane to me and it does work in my testing, but please let me know if it's not the right approach.