- Add an Evas backend for the gui
  
- Auto update of the launcher when we change the Design in config.
  For the moment it doesn't work because the launcher stays open when we start the config application

- Label.optimal_size should be automatically computed when we change the text
  For the moment I do it a very stupid way (see the label implementation)

- Put Box.border, Box.space in the style (may need some more work first)

- Add more comments. Choose a style for the function help string (I remember a nice python PEP on this)