Add an option for one-time timers

[TheRupp]

Checklist

• [x] I made sure that there are no existing issues - open or closed - to which I could contribute my information.
• [x] I made sure that there are no existing discussions - open or closed - to which I could contribute my information.
• [x] I have read the FAQs inside the app (Menu -> About -> FAQs) and my problem isn't listed.
• [x] I have taken the time to fill in all the required details. I understand that the request will be dismissed otherwise.
• [x] This issue contains only one feature request.
• [x] I have read and understood the contribution guidelines.

Feature description

When creating a new timer in the "Timer"-tab I wish, that I had the additional option to create a one-time timer. "One-time" means, that this timer is only used one single time and then disappears automatically. It does NOT stay in the list of saved timers in the "Timer"-tab.

Why do you want this feature?

• I want this feature, because sometimes I only need/want a timer once. Deleting such a timer costs time.
• I personally find one-time timers more intuitive/natural to use than a list of saved timers.
• In my opinion a one-time timer matches the general simplicity of all Fossify apps better than a list of saved timers.

Additional information

Relevant: https://github.com/FossifyOrg/Clock/issues/75