[FEATURE] setting for end minute

[rdmueller]

add setting for end minute.
If not set, it will be calculated through the duration (added to now or start)
If set, use it as minute of the hour where to end the timer.
The start will be the current time or calculated with the duration.
This enables the setup of a timer which ends at a given time - helpful for a count down. For instance when people are in a waiting room, waiting for a session to start.
parameter should be just end

For the three parameters start, end and duration

• if specified only one, the remaining two should be calculated by using the given one and the current minutes of the clock
• if specified two, the remaining one should be calculated by using the given two and the current minutes of the clock