Configure validation error message for each form element

Description

Previously, validation messages for Form elements could only be configured via XML files, and only developers could change those files. And a validation message was used for all element instances that applied the validator.
For example, there are two required elements, First Name and Last Name. The message “This field is required” is used for both; editors could not configure specific messages like: “First Name is required”, “Last Name is required”.

This feature allows editors to customize each form element's validation message in editor mode. Also, custom messages can be language-specific (that is, localization is supported).