Dynamic wrapping for long names for better layout rendering in `legend_tree.py`

In : https://github.com/ecolabdata/secateur/blob/main/core/export/pdf/legend/legend_tree.py

`wrapped_name = "\n".join(textwrap.wrap(node.name(), width=100))` -> should depend on the .qpt template font size, instead of just `100` magic number.