Bug fix: Crashes with drag-drop in bookmarks

Bug fix: Crashes with drag-drop in bookmarks

Also ameliorates https://github.com/BestImageViewer/geeqie/issues/377

bookmark_drag_begin:
eliminate a run-time error message

bookmark_dnd_get_data:
uri_filelist_from_uris returns a list of type FileData. What is required
is text.

bookmark_drag_set_data:
uris_from_filelist expects a list of type FileData, but was given a list
of type text

The code: "gdk_drag_context_get_dest_window(context) ==
gtk_widget_get_window(bm->widget)"
does not function as expected. The reason is unknown.
In order to avoid crashes, drags from bookmarks are disabled for the
moment.
There is no significant loss of functionality.