gluon-web-model: resolve hang when submitting form with disabled element

I was made aware of a bug when submitting the form while the element is
disabled based on it's dependencies

The fix was to inherit from AbstractValue instead of just node

AbstractValue's AbstractValue:resolve_node_depends() in particular
solves the issue, but it made more sense to just use the full base class

package/gluon-web-model/luasrc/usr/lib/lua/gluon/web/model/classes.lua

@@ -410,11 +410,11 @@ function TextValue:__init__(...)
 end
 
 
-local Element = class(Node)
+local Element = class(AbstractValue)
 M.Element = Element
 
 function Element:__init__(template, kv, ...)
-	Node.__init__(self, ...)
+	AbstractValue.__init__(self, ...)
 
 	self.default   = nil
 	self.size      = nil