[696] Improve timer system (time slices + override)

[blitzkneisser]

It would be nice to not use setpoints for timers (be it switch or thermostat) but status time slots.
E.g. not to set a switch on at 16:00 and switch off at 18:00 but specify it in that way:

off from xx:xx to 16:00
on from 16:00 to 18:00
off from 18:00 to yy:yy

Also give a possibility of either falling back to the schedule or at least to read the value that is supposed to be set on the device currently or at any given time.
This would bring 2 major improvements:

First of all, if domoticz - or a device - is offline at switching time, be it because of an error or for maintenance, one has to manually set the device to the state defined at switching time. With the implementation mentioned above, a device would get it's currently desired status the moment it gets online again.

Secondly, it would make advanced skripts easier to implement. For example I have a temperature schedule for my bathroom. Now I want to have some additional device (be it a switch, motion detector or whatever) that raises the temperature e.g. for 30 minutes at activation, afterwards it should fall back to the programmed schedule.
At the moment, this can only be achieved to implement the complete timer within the script, which works but is much more complicated, for changing the schedule, the script has to be modified.
It would be nice to have a script for just implementing the override function and have some command to fall back to schedule mode immediately after the override is no longer required.
Reading the value before override and setting it afterwards also does not help, when there was a scheduled switching time inbetween, this stored value is no longer valid. And if a scheduled switch occurs during an override, the override functionality is also not given anymore...