Add a dbus API

  + dbus activation service
  + check for update, but don't download anything
  + send signal if there are updates pending
  + how big is the current update
  + descriptions for all images (including translations if available)
  + do the upgrade
  + perhaps a cancel one (perhaps plumbed through to download service)
  + signals for pre-reboot success and failure (code or message)
  + apply reboot

  + METHOD: apply reboot
  + METHOD: check for update, but don't download anything
  + METHOD: do the upgrade
  + METHOD: get details: how big and descriptions including i18n
  + METHOD: perhaps a cancel one (perhaps plumbed through to download service)
  + SIGNAL: send signal if there are updates pending
  + SIGNAL: signals for pre-reboot success and failure (code or message)
  + SIGNAL: download canceled

Darn, I don't remember what this one was for:

+ SIGNAL: signals for pre-reboot success and failure (code or message)

so until we need it, I'm skipping it.

After further discussion and thinking, the success case is covered by the ReadyToReboot signal, which gets triggered when GetUpdate() successfully completes.

The failure case isn't really covered, although when the download fails due to a Cancel(), a Canceled() signal is issued.

So I think there is room for a UpdateFailed() signal for when the GetUpdate() fails for reasons other than being canceled. It can have an optional message string, although I'm not sure how helpful I can make that right now.