Add `pip install cdl[qt]` to install DataLab with Qt

[PierreRaybaut]

We could add another option to install DataLab using pip:

• Install DataLab with Qt: pip install cdl[qt]
• Install DataLab without Qt (i.e. Qt bindings are already installed on your Python environment): pip install cdl

Originally posted by @PierreRaybaut in #53 (comment)