`ImageItem`'s `file` is returning `""` as its `path` for single-filed
worktrees like the ones are created for the images dropped from the OS.
`ImageItem::load_image_metadata` had used that `path` in FS operations
and the other method tried to use for icon resolving.

Rework the code to use a more specific, `worktree::File` instead and
always use the `abs_path` when dealing with paths from this `file`.

Release Notes:

- Fixed images not opening on drag and drop into the editor