Addition to Alert Preferences for Bouncing the Dock Icon

I think this is a rather important addition as it allows users to configure when the dock icon bounces and when it does not.

Note before you wonder why the code doesn't 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.