Save color ramps

[JoerivanEngelen]

Fix https://github.com/Deltares/imod-qgis/issues/49

Implement loading and saving of colors. UniqueColorWidget behaves differently than PseudoColor

PseudoColorWidget

• Colormap is saved in colormap file, using QgsRasterRendererUtils. This stores colors and corresponding values.

UniqueColorWidget

• Classify is no longer called with each selection change.
• Only colors saved in json text file, as values are often very unique (e.g. name of borehole), where the user is expected only to be interested in loading colors. In this case colors are loaded and set as the colorramp.
• When n_colors exceeds n_unique_values, colors are cycled instead of repeated. For example if n_colors = 3, n_uniquevalues = 5, formerly this would return [red, red, blue, green, green], this changeset returns [red, blue, green, red, blue]_.