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.

include/xcb.h

@@ -94,7 +94,7 @@ void fake_absolute_configure_notify(Con *con);
  * Sends the WM_TAKE_FOCUS ClientMessage to the given window
  *
  */
-void send_take_focus(xcb_window_t window);
+void send_take_focus(xcb_window_t window, xcb_timestamp_t timestamp);
 
 /**
  * Raises the given window (typically client->frame) above all other windows