the motivation here was to follow the XSMP spec more closely and allow
application's to set custom properties.  the trigger for this was
GNOME's new-ish _GSM_DesktopFile property, which let's an app associate
a .desktop file with itself, so the SM (or other apps) can show a
localized app name, icon, etc.

instead of keeping track of each of the "official" SM properties, we
just have a GTree of everything the client has set.  this makes a lot of
code more generic, and also now makes it easier to make XfsmProperties
look more like a semi-opaque object.  it also removes a lot of annoying
conversion code from XfsmClient, since XfsmProperties uses GValues
internally now.

along with the _GSM_DesktopFile support, xfce4-session now first tries
to use the app's localized name in the splash screen before falling back
to a guess based on SmProgram.