Addition to Alerts Preferences for Bouncing the Dock Icon

I think this is a rather important addition as it allows users to configure when the dock bounces and when it does not.

Note before you wonder why I didn't check to see if the window is in focus for the preference named "Disable notifications when Textual is in focus." the `NSApplication` `requestUserAttention:` method has no effect when the application is in focus anyways.

I'd love any suggestions how I can improve this pull request.