cookbook: Add recipe for key event handling

cookbook: Add recipe for key event handling

Attached patch contains a cookbook recipe about key press event
handling.

It covers both a simple approach (connecting a callback to a
key-press-event signal which manually analyses the key and
modifiers), and a more complicated one based on a binding pool.

There's also some discussion of the two approaches.

http://bugzilla.clutter-project.org/show_bug.cgi?id=2162

Signed-off-by: Emmanuele Bassi <ebassi@linux.intel.com>