Allow cancellation when installing or updating packages

[jddxf]

[FalkF]

I was trying to implement this, but once you hit updateAll actually all updates start at once. For each update one instance of apm is opened. This did not look right and I found this issue: https://github.com/atom/settings-view/issues/756

This also raises the question if it would be ok to abort the apm process, or if that might leave packages in a bad state. So I think cancellation needs to wait until a proper update queue is in place. Maybe with a setting for max simultaneously running apm instances. Then it would be possible to stop starting more updates after the N running updates finished.

Canceling a running update might need changes in apm itself, maybe someone else can tell us more about this?