ChoiceFieldMask form type

Form type to show or hide selected named fields depending on choice selection. This is basically a hack.

Limitations:

* Won't work on nested fields.

Note:  Is not specifc to DoctrinePhpcrAdminBundle - but wouldn't really want it added to AdminBundle as its a hack. 

Making it specific to MenuBundle would be problematic because we can only extend the default form theme template "once" - i.e. in this bundle.