Fixed window.go from the previous commit to improve documentation and change function behavior.
Fixed the implementation of sysData.center() on Windows to be thread-safe; also added center() to the _xSysData interface.
Added Window.Center() test to the test program.
Implemented Window.Center() on Mac OS X.
Implemented Window.Center() on GTK+.
Removed Window.Center() from futureplans.md now that that's done.