SoftwareTimer: make most of the functions virtual
SoftwareTimer: make most of the functions virtual

Make following functions virtual:
- SoftwareTimer::isRunning(),
- SoftwareTimer::start(TickClock::time_point),
- SoftwareTimer::stop().

Remaining overloads of SoftwareTimer::start() are left as non-virtual,
because finally they use SoftwareTimer::start(TickClock::time_point)
anyway.
jasmin-j/distortos,DISTORTEC/distortos,DISTORTEC/distortos,CezaryGapinski/distortos,DISTORTEC/distortos,CezaryGapinski/distortos,jasmin-j/distortos,CezaryGapinski/distortos,jasmin-j/distortos,CezaryGapinski/distortos,DISTORTEC/distortos,jasmin-j/distortos,CezaryGapinski/distortos,jasmin-j/distortos