[feature] Make StopWatchMode not final.

[bastienJS]

I have this use case:

I create a stopwatch with counting up because time passes by and the user is NOT exercising at the moment. When the user starts with exercising the stopwatch should count down if it is a timed based set ;-)

Therefore I need a setter for the StopWatchMode or make it not final. I just need to change very often during user interaction.