ICCCM explicitly states that server time (CurrentTime) should *not* be
used for focus events, but rather the timestamp of the generated event.
Track the last event timestamp and send it down for a WM_TAKE_FOCUS
ClientMessage.  I suspect we should do this for clients that don't
announce this Atom as well, though the raciness gets us into a bind.

Solves focus order issue since WM_TAKE_FOCUS; fix verified by sthen@

ok sthen@
Make sure we really take work area gap into account with snap calculations;
from Dominik Honnef via Christian Neukirchen.