[IMP] web: prevent default in _onActionClicked

This avoids odd behaviour when leaving a `href="#"` on a link with
target=action: without preventing the default behaviour, it would first
switch to the application switcher (/web/#) then actually execute the
action.

addons/web/static/src/js/views/abstract_controller.js

@@ -470,6 +470,7 @@ var AbstractController = mvc.Controller.extend(ActionMixin, {
                 additional_context: _.extend({}, data.context)
             });
         }
+        return false;
     },
     /**
      * FIXME: this logic should be rethought