Ignore false GObject property change notifications

This is related to bug 698275, which did not cover all cases.
The problem here is that the dconf can in certain situation claim
that everything changed (path "/" changed), which GSettingsBinding
propagates to a GObject property unconditionally and GObject's
property setter (g_object_set_property()) also notifies about
the property change unconditionally, despite the real descendant
property setter properly checks for the value change. After all
these false notifications a callback on "notify" signal is called
and possibly an expensive operation is run.

Checking whether the value really changed helps in performance, for
which were added new e-util functions:
   e_signal_connect_notify()
   e_signal_connect_notify_after()
   e_signal_connect_notify_swapped()
   e_signal_connect_notify_object()
which have the same prototype as their GLib counterparts, but they allow
only "notify::..." signals and they test whether the value really changed
before they call the registered callback.