[UI] Support Alerts and Messages

[edmondchuc]

SHACL UI should support a way to display alerts and messages in the form. These messages can be both positive and negative and potentially sourced via the validation report and provide a vocabulary to customise icons, styling, and colour of the message.

Discussion:

• Clarify where these alerts and messages are displayed in the form
• Are they tied to the validation report directly?
• Can this be reused outside of form editing?

[edmondchuc]

Some initial comments on this topic by Thomas Francart. #523 (comment)

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Even before talking about a vocabulary to customise messages, the same apply to shapes : how to associate an icon, a CSS class or a color to a shape ?