Event Every n seconds: Allow one decimal

[Taitava]

No discussion, as this is quite a small change.

Allow one decimal in the Seconds field. E.g. 0.1 would be 100 milliseconds, and 1.1 would be 1100 milliseconds.

This will be in line with a Cooldown setting introduced by #380, which is also seconds and supports one decimal.

• [x] Add to CHANGELOG.md

[Taitava]

Done. Will be released in 0.22.0.

[Taitava]

This is now released.