- add various error/warning messages according to Figma (where it makes
sense)
- letters or more than 2 decimals are caught by the internal validator
so those combinations are impossible to enter
- fix marking the custom value as (in)valid
- fix selecting "Use default" after typing a custom value
- fix resetting to predefined value after typing a custom value that
matches one of the predefined ones
- reject typing thousands separator
- add regression QML tests for the above fixes
Fixes#15017
