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...to check for collisions when the user define a custom shortcut.

...and allow cancelling the IO operations.

The function just returns the no-child flag value, it doesn't
check if the iter has no child.

Use gth_file_source_get_drop_actions to check if a paste operation is
possible.

Allow to move a catalog into a library, and a library into
another library; show an error if the drop operation is
not possible.

...to clarify they are related to the folder tree.

Removed the file argument, which was unused.

It's not used anymore.

It was almost the same widget.