Improved update checking behavior

[tas50]

The current auto-update behavior is fairly basic and needs to be enhanced to improve the number of users that upgrade to the latest release.

Current Behavior

• At launch and periodically the application checks for updates
• The tray application changes the icon when an update is available
• Menu bar has "Download Update" entry which launches web browser to download the app

Desired Behavior

• At launch and periodically the application checks for updates
• The application presents a dialog alerting the user to a new version along with release notes
• The user can select skip this version or update
• Update downloads the version with a progress bar in the window
• Application silently runs the installer

[tas50]

Holding on this overall epic until we have UX designs for the new behavior