v2.3

Another bug fix release for the "Keep display live" feature.

The original design was to have the Time display stay live if the user set the "Keep display live" option, and to have the other tabs (Converter and Info) dim and go dark as normal. That turned out not to work reliably -- iOS calls the appear/disappear closures for the tabs at unpredictable times, which led to apparently random behavior. So, now the display behavior changes exactly when the user changes the default in the Settings app for MultiClock, and affects all of the tabs.