Admin setting abstraction to allow setting to be hidden/disabled when dependent on other settings

There are cases where some settings do not make sense when another setting is disabled - e.g. forum has rss settings which do not make sense when rss is disabled.

At the moment a pattern used is:

if ($CFG->enablerss) { 

  $ADMIN->add(...);

}

The problem with this approach is that the settings inside the if then are not defined at all in the tree and using a setting added this way can cause notices if not using empty()/isset() to check for its value.

Note that we have a way to avoid admin_settingpage() to be displayed (but still defined in the tree) and it would be useful to have more abstractions like that - for example, a setting section.