Many changes:

#1 : Knobs are all passed around with a boost::shared_ptr now. It’s safer and should remain so.
#2 : Settings are now using the Knobs interface and QSettings for serialization/deserialization.
#3 : First implementation of the PreferencesPanel, reflecting the state of the settings of the application.
#4 : Implementation of the colour picker gui: click the label of a colour knob, and then cmd click on a viewer to pick a colour.