Point to versioned help docs

xfdesktop-settings now points to the versioned docs hosted at
docs.xfce.org. This version string will always point to the
major.minor of xfdesktop so it will auto-increment in future relases.
Make xfdesktop-settings pluggable again (Bug 10714)

This changes the X-XfcePluggable back to true and forces wnck to
initialize instead of waiting for it in the main loop, but only
when the dialog is embedded. This allows xfdesktop-settings to
be embeeded in the settings dialog again.