Add the Issues pane
1. Add a class to represent the validation issue
2. Extend the data model for an validation issue to
(1) add a severity level field (useful for identify save-blocking issues later)
(2) replace single entity with an array of entities (useful for issues involving multiple entities)
(3) add a coordinates field for highlighting the location of the issue on the map
(4) add a fixes field for possible automatic fixes
3. Update existing validation modules to use the new data model
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