src: Move visual indication from pwm_modifyLdrBrightness2pwmStep() to user.c

This moves the visual indication to the user to user.c. Instead of only
toggling the PWM output in case a change in offset has been detected, this
makes also sure that the PWM output is toggled each time the appropriate
user command is received.
src: pwm: Introduce functionality to get status of pwm_is_on
src: Move datetime functionality into own module
src: uart_protocol.c: Make use of new datetime module
src: user: Make use of datetime module
src: i2c_rtc.c: i2c_rtc_write(): Remove datetime validation

This removes the datetime validation within i2c_rtc_write() as new date and
time information is expected to be set by datetime_set(), which validates the
input for itself already.
Merge branch 'master' into feat/datetime
Merge branch 'feat/datetime'