[libqofono] Added QOfonoObject and 'valid' property

QOfonoObject encapsulates asynchronous properties query and property change
signal handling. This is a common pattern for many ofono objects.

The 'valid' property on the affected objects becomes true after the initial
GetProperties call completes. That way the user of the object can find out
that the object has finished initializing and its properties actually mean
anything.