[fix] VirtualPage

[ibelar]

Fix for #1587

The callback Callback::set() method needs to be called prior to VirtualPage::getHtml() method in order for other's callback inside VirtualPage to be execute properly.

BC Break

• Property $fx was remove and no longer necessary because Callback::set() method is called during VirtualPage::set() and not during VirtualPage::getHtml() as before. This is to ensure that Callback::set() is always run.

  Therefore, using DI to set $fx property viaVirtualPage::addTo($app, ['fx' => function(){}])is no longer supported.

• Virtual::Page::set() only accept \Closure instead of callable as before. You must convert your callable into Closure via \Closure::fromCallable()

[DarkSide666]

LGTM