[Feature]: Copy to folder: an option to move instead of copy #856

What would you like Filee to do?

A per-slice option on the Copy to folder preset (#814): Move instead of copy. Off by default. When it's on, the files dropped on that slice go into its folder and are gone from where they were.

This is the follow-up mentioned in #821: "copied (no move, to avoid accidents; a per-slice 'move' can be discussed later)".

Why / when would you use it?

Sorting: the case that shaped #821 (downloads and files going into many project folders from the donut). When the point is to file things away, copying leaves the originals behind in Downloads or on the Desktop, and they have to be deleted by hand afterwards, one by one. A slice set to move files them in one gesture.

Ideas for how it could work

Everything else stays as #821 agreed; only what happens to the original changes. To keep accidents away:

• Off by default, per slice: a check box in the Copy to folder preset's editor ("Move the files instead of copying them"). A moving slice could show it in its caption ("Move to Projects") so it's never a surprise.
• Never lose a file:
  • same drive: a plain rename (File.Move / Directory.Move);
  • another drive: copy to a temporary name in the destination, check it's complete, rename it into place, and only then delete the original;
  • a clash follows the existing rule (a number is added, nothing is overwritten);
  • any failure leaves the original where it was, with the message in the toast.
• Same edge cases as Copy to folder: a missing destination fails without creating it; the file's own folder does nothing; a folder into itself is refused.
• Browser drops: there's no original to remove, so a moving slice saves them exactly as a copying one does.
• Ask every time: the save dialog as today; the file is moved to the chosen place.

I have this built on a branch (from an earlier "Actions" idea, before #814 landed) and would rework it onto the Copy to folder preset in this shape, with tests for each of the cases above.