[DESKTOP] Desktop app auto update

[sfroment]

After trying to use AutoUpdater through astilectron I've been stuck by the squirrel.mac signature verification of the .zip
Using autoUpdater from electron should be great but it's a waste of time for the moment here are the reasons why:

• we will need to add to astilectron and go-astilectron a way to call autoUpdater this part is the easy part
  In https://github.com/asticode/astilectron/blob/master/main.js#L85 you need to add event that will be send from the go to the electron

     case consts.eventNames.autoUpdaterCheckForUpdates:
        autoUpdater.checkForUpdates()
        client.write('1', consts.eventNames.autoUpdaterCheckForUpdates, {})
        break
      case consts.eventNames.autoUpdaterGetFeedURL:
        client.write('1', consts.eventNames.autoUpdaterGetFeedURL, { url: autoUpdater.getFeedURL() })
        break
      case consts.eventNames.autoUpdaterSetFeedURL:
        autoUpdater.setFeedURL({ url: json.url })
        client.write('1', consts.eventNames.autoUpdaterSetFeedURL, { url: json.url })
        break

You'll also need to listen to the event send by electron to send them to the go https://electronjs.org/docs/api/auto-updater#events
Lastly you need in add a type AutoUpdater in go-astilectron it could look like something like:

package astilectron

type AutoUpdater struct {
	*object
}

func (au *AutoUpdater) GetFeedURL() (string, err error) {
	o, err := synchronousEvent(au.c, au, au.w, Event{Name: "auto.updater.get.feed.url"}, "auto.updater.get.feed.url")
	return nil, err
}

func (au *AutoUpdater) SetFeedURL(url string) (err error) {
	_, err = synchronousEvent(au.c, au, au.w, Event{Name: "auto.updater.set.feed.url", URL: url}, "auto.updater.set.feed.url")
	return
}

func (au *AutoUpdater) checkForUpdates() (bool, err error) {
	_, err = synchronousEvent(au.c, au, au.w, Event{Name: "auto.updater.check.for.updates"}, "auto.updater.check.for.updates")
	return
}

func (au *AutoUpdater) QuitAndInstall() (err error) {
	_, err = synchronousEvent(au.c, au, au.w, Event{Name: "auto.updater.quit.and.install"}, "auto.updater.quit.and.install")
	return
}

• we will need to make a new electron provisioner to ship electron and astilectron in our .dmg
• we will need to sign electron and the electron helpers
• we will need to dev an api compliant we've the squirrel.mac requirement such as https://github.com/Aluxian/squirrel-updates-server

I think it will be great to use AutoUpdater once we're open source and we make release based on github releases

#1176