feat: add support for state groups for the Remove relation

[pancsta]

Ability to define a mutually exclusive state group and using it for the Remove relation for all the states from that group.

Reduces repetition, increases readability.

var GroupPlaying = am.S{
	Playing, Paused, LiveView,
}

var States = am.States{
	Playing: {
		Remove: GroupPlaying,
	},
	Paused: {
		Remove: GroupPlaying,
	},
	LiveView: {
		Remove: GroupPlaying,
	},