Put some things in the Transferable group

[ddbeck]

Originally suggested in #987 (comment). I grouped any non-draft that contained a transferable object, to see how things looked.

@foolip originally wrote:

My hunch is that not all of these things will be in features by themselves, and the features they are part have some other primary location in a feature group hierarchy, like MediaSourceHandle being a part of Media Source Extensions or some media group.

My feeling is that canvas is probably rightly grouped here, but the other two are much more uncertain.

(I've been on a tab-closing spree today, so apologies if this seems out of the blue.)

Closes #1512

[foolip]

This group strikes me as quite odd, a bit like having a group for any APIs that use promises. Are you sure it'll be useful for someone to filter features by "includes something transferable"?

[ddbeck]

Are you sure it'll be useful for someone to filter features by "includes something transferable"?

Nope! I just saw the suggestion on the previous PR and wondered what it would look like. I think it would be totally find to get rid of the transferable group entirely. #1512 is what that would look like.