Discord Rich Presence integration

[aoki-marika]

Adds optional Discord Rich Presence integration.

I used a temporary Discord Application on my own account for testing, but omitted it here. It would make more sense for one of the maintainers to create an application on their account so it can be properly managed.

Note that, as I'm no designer, this contains no assets. Images are needed for the large white box (the game's icon) and the smaller white circle (the current mode; drums or guitar). Each must be at least 512x512 and square. I'm unsure who can provide them though, perhaps the creator(s) of the included skins?

Resolves #41.

[ericpignet]

Hi @aoki-marika great job.
I created a Team DTXMania on Discord developer website with you and @limyz as co-admins, and an app DTXManiaNX.
Can you check if you have access and use it for IDs?
I'll see if I can get something simple done for assets, starting from the Nextage skin. Better designers can improve it later.

[ericpignet]

I added a commit with the ID and image keys from new DTXMania NX Discord game, then merged.
Thanks again @aoki-marika !