Send last event timestamp with WM_TAKE_FOCUS msg

Send last event timestamp with WM_TAKE_FOCUS msg

According to 4.1.7 of the iccm spec

http://tronche.com/gui/x/icccm/sec-4.html#s-4.1.7

> Windows with the atom WM_TAKE_FOCUS in their WM_PROTOCOLS property may
> receive a ClientMessage event from the window manager (as described in
> section 4.2.8) with WM_TAKE_FOCUS in its data[0] field and a valid
> timestamp (i.e. not CurrentTime ) in its data[1] field.

Adds the timestamp parameter to send_take_focus to avoid the dangerous
use of a global variable.