Palette integration in the legend editor

Integrates palettes in the legend editor. The code was originally taken from openJUMP and then simplified and documented. We have a class representing palette that is used later on in the interface.

I've extracted some code from PnlAbstractCategorized in a dedicated class : ColorConfigurationPanel. This offered me more flexibility without duplication... This class builds a panel that contains the gradient and palette configuration associated to a radio button that can be used to choose the enabled inner panel.

The previous code for color generation has been improved. Computation now relies on palettes. When using a gradient, we first create a palette with the two user-defined colours and then we compute the missing ones. When using a richer palette, colours from the palette have priority. If we need more, we compute the missing one in the intervals of the input palette, as we do for gradients.

Palettes have been integrated in the UI. Note that it is currently not possible to add external palettes. This feature will be added later...

Finally, I've replaced the text field used until now with a combo box ( see #449 ) and I've fixed a bug with the quantile implementation (see #451). (Note that i've confused these two issues in one of my commit, my bad...).