To enable the specification of key names in configuration files parsed
at runtime, we need the association between key-code values and their
respective names.
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
