Add comments to Watch::frame() tick compensation

See previous commit made.
Move Watch impl. to .cpp file
Make min, hour etc. increments circular

That is when incMinutes() is called it will set minutes to 0
provided that it was 59 prior to the call.
This fixes assertion failure raised in checkInvariant()
Meld Display into Watch class

Display currently is very thin, don't want to maintain it
Make Time display depend on 24/am-pm flag