Update for dbus-sharp 0.8.1 & dbus-sharp-glib 0.6

We now require both dbus-sharp 0.8.1 and dbus-sharp-glib 0.6.

Switching to >=0.8 dbus-sharp was necessary as a first step
towards fixing bgo#630110 ("Get single instance and remote
control working on Win32"). But it's rather better to already
require 0.8.1 instead of 0.8, because 0.8 had regressions (see
bgo#725446).

These new versions of these dependencies have API/ABI breaks, so
then their version in their pc files is raised from 1.0 to 2.0.

Switching to these versions allow us to also make some cleanups:

We were using an obsolete Bus.Session.Register which required a bus
name, and it's now gone. This allows us to remove some
DBusServiceManager.RegisterObject overloads that become unnecessary.

As an additional consequence, we don't need the DBusExportable attribute
anymore. It was used by classes implementing IDBusExportable to indicate
their service name, which was then used for the bus name.