Update to MEI5

[rettinghaus]

Now that MEI5 is out we should update the Plugin.
I'm not quite sure how to proceed with this. Should we go simply to mei5 and create another "legacy" branch (as discussed in #169)?

[th-we]

As we have a client who "ordered" MEI 4 from Sibelius input, I'd like to stick with MEI 4 for the things I'm going to do next. I'll be absent for the coming week, but hope to continue with Sibmei work in one or two weeks. One thing I would like to finish is the re-structuring of the "main loop" that I started during the Berlin technical team meeting.

[ahankinson]

Can we create a "stable" branch for MEI4, as suggested by @rettinghaus?

My experience is that it's better to tackle this stuff sooner rather than later, and we're not sure how long your client will want to remain on MEI4. It might be "forever" in which case we would be delaying the upgrade for everybody else.

So it's better for the project to move on, but obviously we don't want to cause you unnecessary trouble. How much more work do you have to do on your refactor? Rough estimates are OK: A week, a month, three months, six months?

[th-we]

@ahankinson I'd say in two months time, I should be done.