Give shows model items knowledge of their position in their parent

Signed-off-by: Simon Marchi <simon.marchi@polymtl.ca>
Add set_children to shows tree model items

Signed-off-by: Simon Marchi <simon.marchi@polymtl.ca>
Call beginRemoveRows to notify that we remove the "Loading" entry

Signed-off-by: Simon Marchi <simon.marchi@polymtl.ca>
CLI: add -f option to allow overwriting
CLI: better Python
CLI: handle `FileExists` exception for multifetch
CLI: order options alphabetically
CLI: order commands logically