Fix WAIT WINDOW when the message follows WINDOW

[Irwin1985]

Fixes #1992

I hope this is the last PR I do on the WAIT WINDOW rule ;-). Every time I patch one, another crack show up somethere else, that's driving me crazy.

I checked every combination against a real VFP 9 and all of these are valid there:

WAIT WINDOW <msg> [AT r,c] [NOWAIT] [NOCLEAR] [TIMEOUT n]
WAIT WINDOW <msg> TO <v>
WAIT WINDOW <msg> AT r,c TO <v>
WAIT WINDOW TO <v>

Fingers crossed.