Add ability to launch a bundle

This commit adds the `launch_bundle` function in `sugar3.activity.
activity` which accesses the same function in the shell is over dbus.
This is means the shell process launches the bundle.  Activities
should not launch child processes as per the Rainbow security model
[1].

[1]  http://wiki.laptop.org/go/Rainbow

src/sugar3/activity/activity.py

@@ -1159,3 +1159,10 @@ def show_object_in_journal(object_id):
     obj = bus.get_object(J_DBUS_SERVICE, J_DBUS_PATH)
     journal = dbus.Interface(obj, J_DBUS_INTERFACE)
     journal.ShowObject(object_id)
+
+
+def launch_bundle(bundle_id='', object_id=''):
+    bus = dbus.SessionBus()
+    obj = bus.get_object(J_DBUS_SERVICE, J_DBUS_PATH)
+    bundle_launcher = dbus.Interface(obj, J_DBUS_INTERFACE)
+    return bundle_launcher.LaunchBundle(bundle_id, object_id)