Add midi output support

[grace125]

Objective

• Create a plugin for sending midi events
• Add an example to showcase the new feature

[BlackPhlox]

Amazing! 🤩 I got time to look at this on Sunday, can you run the cargo fmt command and commit? 😊

[grace125]

Done!

I'm also thinking about submitting a PR to change how MidiInputPlugin works to more closely match MidiOutputPlugin (e.g. provide a more general way to connect to input ports). Let me know if that'd be welcome.

[BlackPhlox]

Sure, I'll look at it in detail Sunday. I think a group plugin would also be nice, grouping both input and output called Midi (thus only needing users to change from .add_plugin(Midi) to .add_plugins(Midi)). Though that might require some to change to both plugins in terms of the connecting process. I would like to hear your opinion on this 😊

[grace125]

I think a group plugin would make sense, but it should probably be named MidiPlugins just to follow the convention of other plugins. Plus, naming it Midi might encourage users updating their code to add MidiOutputPlugin when they don't need it.

[BlackPhlox]

Overall this is some really amazing work and a huge improvement of the ergonimics! Though I think we can improve it even further. Though time is always a factor. I think we should move this pr into a new branch as part of an overhaul instead and then when we think we are done, then it merged in, just like with Leafwing-Studios/leafwing-input-manager#164. What do you think?

[grace125]

I'm down for moving this pr to a branch and working on it there.

Though I just realised that maybe MidiOutput should be a non send resource. I don't really like how I did error handling. Let me try it out first.

[BlackPhlox]

Amazing! I've added the branch 0.6 so that you can now change the target branch of this pr :)