(prototype) multiple elements on the same line

[raboof]

When a note or signal is indented with '=' it is shown on the same height as the latest note or signal without indentation.

This is just a PoC implementation of #130. The formatting is mucked up and the logic is a bit hard to read. If we decide this is a useful feature we probably want to make 'lines' (collections of notes/signals that are on the same line) a first-class citizen.

Would love to hear what you think!