[Style] Allow to style background of state header/title

@startuml
<style>
stateDiagram {
  element {
    BackGroundColor orange
	title {
	  Fontcolor red
	  FontStyle plain
	  BackGroundColor pink
	}
  }
  header {
    BackGroundColor aliceblue
    Fontcolor blue
    FontSize 12
  }
  stateBody {
    BackGroundColor palegreen
  }
}
</style>

hide empty description 

state s0

state "Title ('element > title' style)" as s1 {
  s1: State description (header style!)
  state "s2 (title style)" as s2
  state s3: long descr. (header style!)
  state "s4 (title style)" as s4
  s4: long descr. (header style!)
}

s0 --> s2
s2 -> s3
s3 -> s4
@enduml