API to Notify render processes when updates are available

[mcolyer]

This is a proposal for how we can communicate from the browser process to the render process that a new update is available. This was my best guess about how to do it but I'm definitely open to suggestions.

Summary

• Sparkle triggers 'ready-for-update-on-quit' in the browser process
• The browser process forwards that event as a command to all of it's render processes.
• Each render process receives that event as a command and triggers it on a rootView.
• Packages that would like to know when an update is available listen to window:update-available

Note this branch also adds updateAvailable to loadSettings which is passed to AtomWindow (which is available in a browser process by calling atom.getLoadSettings().updateAvailable).

This enables atom-archive/release-notes#1

[kevinsawicki]

Looks good to me, 👍

[probablycorey]

Instead of keeping state in atom.getLoadSettings().updateAvailable I think it might be simpler to always use the 'window:update-available' IPC event. We can trigger it after the window is created. This will limit the places where the update state is stored.

[mcolyer]

@probablycorey how about now?