Make minimize-to-tray optional

[mvastarelli]

Currently the application will minimize to the tray by default. We should turn this into an optional feature since the icon for MM is not widely known and it could confuse the user to see the application "disappear" as soon as it's been minimized.

[mvastarelli]

To fix the following should be done.

1. Disable this functionality by default in the main process.
2. Add a new app setting to toggle this.
3. Update the Settings screen to display this setting.
4. Add a hook to the main process to react when this setting is changed.

Optionally we could update the handler in the main process to check the app setting whenever the user clicks the minimize button, but I'm uncomfortable with the main window taking a direct dependency on a module.