Alt + R shortcut conflicts with Edit menu shortcut in Swedish version of Firefox

[birtles]

As reported here: https://birtles.wordpress.com/2017/01/10/web-animation-in-2017/comment-page-1/#comment-1305

Once Firefox bug 1303384 is fixed it will be possible for the user to re-assign these commands themselves, but it might be a long time before that happens.

Perhaps for now we could just add a pref to toggle the Alt + R shortcut and alternatively the previous proposed Alt+6 shortcut?

[birtles]

Oh, nice. Not only has Firefox now got a means to update app shortcuts (as of 6 days ago) but Piro has added a library for it!

[birtles]

This has taken a lot of work but I think I'm nearly done.

I've just noticed now, though, that on Windows we don't seem to be calling preventDefault() correctly making it impossible to set e.g.Alt+F11 as the shortcut key.

[birtles]

All the pieces of this are now fixed on master.