And remove some dead code.
Update various places for what we can do with C++20: - ends_with / starts_with - std::to_array - using instead of typedef - designated initializers - "[[maybe_unused]]" instead of G_GNUC_UNUSED Also remove some unnecessary 'static'
Basically, move/rename lib/mu-message* to lib/mu-*. Add the beginnings of a Message class.
