Unify handling of dropping one file or several files on the trash bin

When a single file was dropped on the trash bin the file information was
gotten from the original element in the file list. When several files
were dropped on the trash bin the file information was gotten from the
helper elements being dragged around. The helper element also contain
the needed file information when a single file is being dragged, so the
handling was unified to always get the file information from the helper
elements.

As the handling of several files is the same as before there is still
the issue of only deleting those files shown in the drag helper instead
of all the selected files.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>