Rewrite requestMIDIAccess to hook into permissions

[miketaylr]

As a followup to #219 (review)

[cwilso]

What remains on top of #294?

[miketaylr]

Sorry for not writing a better description!

Essentially, https://webaudio.github.io/web-midi-api/#dom-navigator-requestmidiaccess needs to hook into the Permissions algorithms.

@marcoscaceres wrote some more explicit suggestions in #219 (review).

[miketaylr]

I will attempt to not mess this up.

[hoch]

Teleconference 4/6:
Remove "particularly if system exclusive access is requested." in the second paragraph of https://www.w3.org/TR/webmidi/#requestmidiaccess.

[miketaylr]

I will attempt to not mess this up.

(It looks like I never finished...)

[hoch]

No worries! We'll take it from here. :)

[miketaylr]

Yay, thanks! I'll go ahead and close my WIP PR now.