Use group _context to avoid leaking session context

When launching a new action, the context should not contain the session
context because it is automatically appended to the evaluation.
When creating a new group when the context model changed, it should not
contain the session context neither.

issue8312
review267301002

src/screen.js

@@ -978,7 +978,8 @@
                     this.context_screen.display(true);
                     return jQuery.when();
                 }
-                this.new_group(jQuery.extend(this.context,
+                this.new_group(jQuery.extend({},
+                    this.group._context,
                     this.context_screen.get_on_change_value()));
             }
 

src/tab.js

@@ -644,7 +644,7 @@
                                     ids: record_ids
                                 };
                                 Sao.Action.exec_action(exec_action, data,
-                                    screen.context);
+                                    jQuery.extend({}, screen.group._context));
                             });
                         }.bind(this))
                         .appendTo(menu);