If I'm getting a QScriptValue from e.g. a JavaScript dictionary or array, it
would be good if I could use the [] operator on the QScriptValue to retrieve
data from this value, just like one would do in JavaScript, i.e.:

 from PySide.QtCore import *
 from PySide.QtScript import *

 app = QCoreApplication([])

 engine = QScriptEngine()
 value = engine.evaluate('x = {"a": 1, "b":2}')
 print value['a'] # should be equivalent to value.property('a').toVariant()

Due to the way the engine works, we could then also access array members via a
numeric index (which would be converted to a string for property access):

 value = engine.evaluate('x = ["x", "y", "z"]')
 print value[1] # should be equivalent to value.property('1').toVariant()

In case the property returned is not valid (.isValid() returns False), a
KeyError should be thrown if the key was a string, and an IndexError should be
thrown if the key was an integer.