custom timezone, for world clock that user can add "±1:30", "±2:30" or etc

[0xRustlang]

Checklist

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

Feature description

Please allow user to set custom timezone, for world clock (an option to ignore what have set in OS) that user can add "±1:30", "±2:30" or etc (both negative or positive amount)

Why do you want this feature?

Many countries recently has changed their DST laws and stopped using it and that has created many issues for those that has old devices.

If you add these options, they really save most of annoyances that this caused users and really help them.

Thank you

Additional information

(I also had another request for exact same option for "main clock" part of app also but as you said that every issue should be about one request, i had created #36 for that and link it into this, if you want, i can merge them later)