Redesign of Alerts tab of Preferences.

This was done to make the user more aware that the settings presented on the pane are those of the selected alert.
This commit removes the option to disable notifications when Textual is in focus. Instead, this setting is always used.
Notifications will not be posted while Textual is in focus.
Do away with these settings. They have been broken for a long time and now are considered officially gone.
Graduate the “toggle away on display sleep” option. It is now available under the General tab.
Retire setting. Feature is mature. No reason user should need to disable.
Also retire this option. It can be used, always. No need to turn off anymore.
More UI touchups.