[FEATURE]: fixed updates-times

[Stopuhr]

Edited in agreement with ace

Actually inkcal is started manually or at boot and is refreshed at configured period.
Even in normal usage it would be better to update once at startup and from that on in at configured times. So you can be sure not to interfere with calibration or other processes.

In my case, the update should only by at night, so that the agenda is actual all day and display the flickering update is not taken as a fault.
By now I have to plan reboot at night, if I had to change something.