Add engine version identifier and expose this ID over the DBus API

This is a standardized DBus Properties interface we can use for this...

We might also want to version our DBus interfaces (like fx. Tracker does since 0.7), but that is not exactly what this bug is about...

I implemented a Properties interface in lp:~thekorn/zeitgeist/dbus.properties
The 'version' property is a read-only property which returns a version tuple.

Example usage:

>>> from zeitgeist.dbusutils import DBusInterface
>>> iface = DBusInterface()
>>> iface.version()
dbus.Struct((dbus.Int32(0), dbus.Int32(3), dbus.Int32(0)), signature=None, variant_level=1)
>>> map(int, iface.version())
[0, 3, 0]
>>> iface.version() > (0, 1, 0)
True
>>> iface.version() > (1, 5, 0)
False

Or in a terminal

markus@thekorn ~ % dbus-send --print-reply --dest=org.gnome.zeitgeist /org/gnome/zeitgeist org.freedesktop.DBus.Properties.Get string:"org.gnome.zeitgeist" string:"version"
method return sender=:1.170 -> dest=:1.171 reply_serial=2
   variant struct {
         int32 0
         int32 3
         int32 0
      }
markus@thekorn ~ % dbus-send --print-reply --dest=org.gnome.zeitgeist /org/gnome/zeitgeist org.freedesktop.DBus.Properties.GetAll string:"org.gnome.zeitgeist"
method return sender=:1.179 -> dest=:1.180 reply_serial=2
   array [
      dict entry(
         string "version"
         variant struct {
               int32 0
               int32 3
               int32 0
            }
      )
   ]
markus@thekorn ~ % dbus-send --print-reply --dest=org.gnome.zeitgeist /org/gnome/zeitgeist org.freedesktop.DBus.Properties.Set string:"org.gnome.zeitgeist" string:"version" string:"booo"
Error org.freedesktop.DBus.Python.AttributeError: Traceback (most recent call last):
  File "/usr/lib/pymodules/python2.6/dbus/service.py", line 702, in _message_cb
    retval = candidate_method(self, *args, **keywords)
  File "/media/devel/gnome-zeitgeist/zeitgeist/dbus.properties/zeitgeist/../_zeitgeist/engine/remote.py", line 265, in Set
    raise AttributeError(property_name)
AttributeError: version

1 markus@thekorn ~ %

Targeted to 0.3, because of the to be expected API change.

I think it looks really good. Easy to extend with new properties, readable, as well as writable. You should propose the branch for a merge.

Merge proposal is at https://code.edge.launchpad.net/~thekorn/zeitgeist/dbus.properties/+merge/14188

Mikkel, did I implement the DBus Properties correctly, is it following your proposal in comment #1?

I think this is done for right?