Allow controlling the display of a Dashboard Gadget from its content

Right now it's not possible to have a gadget dynamically deciding if it should be displayed or not.

Example use case: The Message Sender gadget on the XE default dashboard should only be displayed if the Message Sender feature is active.

The issue is about implementing the following:

• When a gadget's rendered content is empty, don't display the gadget

This means executing rendering macros in the Dashboard macro vs executing them when the page is rendered. It just means that the order of the macros execution of the macros located in gadget may change.