New Input::Event::FOCUS, rename keycode to code

This patch introduces keyboard-focus events to the 'Input::Event' class
and changes the name 'Input::Event::keycode' to 'code'. The 'code'
represents the key code for PRESS/RELEASE events, and the focus state
for FOCUS events (0 - unfocused, 1 - focused).

Furthermore, nitpicker has been adapted to deliver FOCUS events to its
clients.

Fixes #609