Extend data model for validation issues

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

modules/core/history.js

@@ -281,9 +281,12 @@ export function coreHistory(context) {
 
 
         validate: function(changes) {
-            return _flatten(_map(Validations, function(fn) {
-                return fn(context)(changes, _stack[_index].graph);
-            }));
+            return _flatten(_map(
+                _filter(Validations, _isFunction),
+                function(fn) {
+                    return fn(context)(changes, _stack[_index].graph);
+                }
+            ));
         },