Create separate fields in X screen structure to hold visibility,

Create separate fields in X screen structure to hold visibility,
whether window is mapped, and whether window is exposed.  Synthesize
SCREEN-VISIBILITY from these fields; previously SCREEN-VISIBILITY held
all this information and was sometimes inaccurate.

Don't attempt to give focus to an unmapped frame.  This will only
signal an error.

Don't map an unmapped frame when entering it.  Eliminate variable
X-SCREEN-AUTO-RAISE and associated command.  Both of these things were
window management; we shouldn't be doing that.