wplua/sandbox: support mixing static methods and constructors in class identifiers

Global class identifiers, such as "Node", "SessionItem", "Conf", etc
are so far defined either as methods, which are constructors for
the GObject class, or as tables, which contain "static" methods, i.e.
methods that can be called without an instance.

In some cases, we may want to mix a class being both instantiatable
with a constructor and also have static methods. To support this,
allow the class identifier be declared as a table with the constructor
being defined as the "__new" method. This change allows calling the
table as a method and execute "__new" underneath.

For instance:
```
json = Conf.get_section_as_json("foobar") -- static method
conf = Conf("/foo/bar") -- constructor, equivalent to Conf.__new("/foo/bar")
```

See also !629

modules/module-lua-scripting/wplua/sandbox.lua

@@ -43,6 +43,9 @@ function create_sandbox_env()
       if type(v) == "table" then
         SANDBOX_ENV[k] = setmetatable({}, {
           __index = v,
+          __call = function(t, ...)
+            return t["__new"](...)
+          end,
           __newindex = function(_, attr_name, _)
             error('Can not modify ' .. k .. '.' .. attr_name .. '. Protected by the sandbox.')
           end