backup & restore: subplugins support

Add subplugins support for backup.

Relying on corresponding "subplugin.php" file for modules / declaring them manually via PHP API:

$element->register_subplugin(whatever);

No custom handling allowed by plugins in order to handle subplugins. It's a core supported feature and core backup will treat all them exactly the same way.

Use optigroups extensively (both unique and multiple ones - the last for the workshop forms at least).

Ciao