Time Signatures on top of the score using a separate staff

In 20th century music, where time signatures tend to change a lot, it is sometimes recommended to put the time signatures on top of the score (or above each StaffGroup in the case of an orchestral score). This can be achieved by creating a dummy staff which only contains the Time_signature_engraver.

In this specific example, a separate identifier is used to define every time signature change, which saves the bother of entering them again when typing the actual music (careful though: it makes getting lost easier!).

Notice the overriding of the X-offset property: a specific trick to make the time signatures align with the barlines, thanks to Han-Wen.