Implemented lilypondnametools.LilyPondContextSetting.

LilyPondContextSetting, like LilyPondGrobOverride, provide an object-oriented
interface to LilyPond context setting and grob override formatting logic.

They can be used to generate \set and \override string in lieu of building
those string by hand. LilyPondContextSetting can also be attached to score
components, like LilyPondGrobOverride.