Use new libxml2 buffer API

This reflects the changes in libxml2 2.9.0 described here:

https://mail.gnome.org/archives/xml/2012-August/msg00005.html

Liferea compiles against 2.9.0 without these edits, but gives compile
time warnings that xmlBufPtr is passed where xmlBufferPtr was
expected.  These structs are binary compatible, so it also runs fine.
But fixing it is of course preferred.

Signed-off-by: Lars Windolf <lars.lindner@gmail.com>
ChangeLog for patch SF #224: Update to new libxml2 buffer API (Simon Kagedal Reimer)
Improving comments in feedlist_schedule_save_cb()
Removed '(GNOME Classic)' as it works with GNOME shell too.