Autopilot does not honor the object cache when creating the root introspection object

Currently, if a user defines a custom proxy class for the node at the root of the introspection tree, it does not get returned by the launch_* methods.

The reason for this is that, in _search.py, the _get_proxy_bases_from_introspection_xml does this:

---
def _get_proxy_bases_from_introspection_xml(introspection_xml):
    bases = [ApplicationProxyObject]
---

and ApplicationProxyObject is defined (just a few lines further down) as:

---
class ApplicationProxyObject(ap_dbus.DBusIntrospectionObject):
---

So, the root proxy object will never pick up any CPC's in the object registry. To fix this, let's make ApplicationProxyObject a simple mixin class:

---
class ApplicationProxyObject(object):
    """A class that better supports query data from an application."""

    def __init__(self):
        self._process = None
---

and then use the existing methods defined in autopilot.introspection._object_registry to get the appropriate proxy class:

---
def _get_proxy_bases_from_introspection_xml(introspection_xml):
    bases = [ call_to_get_proxy_class(...), ApplicationProxyObject]
---

I'm too lazy to figure out what the exact call should be, but I'm sure it exists in the _object_registry module. This will probably involve changing the _get_proxy_bases_from_introspection_xml function to take some additional parameters, or some refactoring to pass in the base class.

The test case for this should be:

Write a simple Qml app script, and define a custom proxy class for the root node. Assert that:

* When launching the app, we get an instance of the CPC, not a generated class.
* When calling the get_root_instance call on any other proxy object, we get an instance of the CPC, not a generated class.