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.

src/x.c

@@ -992,7 +992,7 @@ void x_push_changes(Con *con) {
                 focused->window->needs_take_focus) {
                 DLOG("Updating focus by sending WM_TAKE_FOCUS to window 0x%08x (focused: %p / %s)\n",
                      to_focus, focused, focused->name);
-                send_take_focus(to_focus);
+                send_take_focus(to_focus, last_timestamp);
                 set_focus = !focused->window->doesnt_accept_focus;
                 DLOG("set_focus = %d\n", set_focus);