Can't use the velocity macro inside Dashboard gadgets

After the introduction of the user dashboard feature (XWIKI-6820), the Dashboard.WebHome page started using the activity macro inside a velocity macro. This had the effect that all gadgets trying to use the velocity macro for themselves got a "Nested script exception" since a velocity macro inside another velocity macro is not allowed.

To fix this, the Dashboard java macro needs to implement NestedScriptMacroEnabled to allow inner velocity macros (inside gadgets).