Insubstantial Issue #68: BaseColorScheme API too limiting

Reworked the derived colors and BaseColorScheme to use immutable, flyweights and forward resolvers to derived color schemes.

This change introduces new API:

SchemeDerivedColorsResolver is similar to SchemeDerivedColors, but each method takes a SubstanceColorScheme. This allows implementations to create classes that are preferably immutable, but, at the very least, stateless. Note class documentation asks implementations to be immutable.
BaseColorScheme gains a new protected constructor that takes a SchemeDerivedColorsResolver. This allows derived color schemes to reuse the same color resolver.
Both the original BaseColorScheme constructor and the new SchemeDerivedColorsResolver one defer to a third private constructor.

This change attempts to maintain backward compatibility:

SchemeDerivedColorsResolver contains an isDark method. This is designed to ease migration for current SubstanceColorScheme implementations.
The BaseColorScheme private constructor helps maintain the backwards compatibility by allowing the isDark nature to be retained.
The new protected static method BaseColorScheme.getResolver aids implementations is determining the current SchemeDerivedColorsResolver and more easily use the new BaseColorScheme constructor.

This change is API breaking in the following ways:

The SchemeDerivedColorsResolver interface (new) replaces the current interface parent of SchemeDerivedColors for DerivedColorsResolverLight and DerivedColorsResolverDark. It also replaces the SchemeDerivedColors for the field BaseColorScheme.derivedColorsResolver.

This change introduces the following unresolved bugs:

Because SchemeDerivedColorsResolver implementations are supposed to be immutable, the current implementations should be made final. One of them, DerivedColorsResolverLight, is public and cannot be made final without adding more API breakage, cf. previous section.

Insubstantial / insubstantial