Import txsgui

This merge request adds the version of txsgui you sent me by email in June to txs under txs.app and updates setup.py to:

• install txsgui as a txs command
• Allow to install GUI dependency with pip install .[gui]

I made minimal changes to make the GUI start (see tvincent/txs@9c0b36c1), but starting a processing fails (some API changed). I'm looking into this.

How do you want to work? With small merge request that are easier to review? --in this case this is ready-- or with a bigger one that is fully working?