open file vs. open files

[andrea-bistacchi]

open_file_dialog in helper_dialogs has a multiple option (default False) that seems to be focused on opening all files in a directory, and not a specific selection of files. This is used just for point clouds and for an old method to import multiple cross sections that will be removed.

I added open_files_dialog for this purpose.

In the future we probably would use the same also for point clouds and maybe other cases.