client: fix focus subsystem

client: fix focus subsystem

We don't need to unfocus currently selected client.
Added WM_TAKE_FOCUS atom, so we can implement correct ICCCM,
focus behavior, in relation to Input Models ( sections 4.1.7 [1],
4.2.7 [2] ).
Currently correctly implemented are, "No Input", and "Passive"
models, and additionally works "Locally Active".
To test focus-in and focus-out events I used program from [3].

[1] http://tronche.com/gui/x/icccm/sec-4.html#s-4.1.7
[2] http://tronche.com/gui/x/icccm/sec-4.html#s-4.2.7
[3] http://mail.gnome.org/archives/gtk-devel-list/2001-November/msg00334.html

Signed-off-by: Mariusz Ceier <mceier@gmail.com>
Signed-off-by: Julien Danjou <julien@danjou.info>