xfce4-session SmRestartImmediately support

[Brian J. Tarricone]

Hi guys,

I just committed SmRestartImmediately support for xfce4-session.
Xfdesktop already uses it.  I'd suggest that xfce4-panel and xfwm4 be
changed to use it as well, if they aren't already.

They'll both need extra changes too:

Panel: If the user runs 'xfce4-panel -x', the panel should first reset
its SM restart hint to SmRestartIfRunning before exiting.

WM: If the user runs 'xfwm4 --replace', the old instance of the WM
should set its SM restart hint to SmRestartIfRunning before exiting.

May want to do the same with 'Thunar --daemon'.  Or maybe not.

I'm thinking about making xfce4-settings-helper session-managed so it
can take advantage of this as well.  Any thoughts about this?  (I have
an idea how to solve the first-time startup issue; will flesh this out
later).

	-brian