new menuing stuff in xfdesktop

[Brian J. Tarricone]

Hi all,

I finally closed a long-standing bug[1] -- the right-click context menu
when file/launcher icons is enabled should be pretty sane now.  If you
right-click on an empty area of the desktop, you'll get the desktop's
context menu (with 'open in new window', 'create launcher', etc.).  For
both file icons and window icons, every context menu that 'replaces'
the standard xfce root menu now has the root menu in a submenu.

When desktop icons are disabled, the old behavior is retained.

Please test the hell out of this, as it required some reasonably large
changes in how menuing is handled in xfdesktop, and it's possible I
introduced a few bugs here and there.

(Related note: I bumped the version requirements for libxfce4menu
because Jannis changed some behavior.  Be sure to update libxfce4menu
first, otherwise, the root menu won't appear at all.)

	-brian

[1] http://bugzilla.xfce.org/show_bug.cgi?id=2531