Move some more shared x-, w32- things to common-win.

Move some more shared x-, w32- things to common-win.

* term/ns-win.el (x-select-text): Doc fix.
* w32-fns.el (x-alternatives-map, x-setup-function-keys)
(x-select-text): Move to term/common-win.
* term/w32-win.el (xw-defined-colors): Move to common-win.
* term/x-win.el (xw-defined-colors, x-alternatives-map)
(x-setup-function-keys, x-select-text): Move to common-win.
* term/common-win.el (x-select-text, x-alternatives-map)
(x-setup-function-keys, xw-defined-colors): Merge x- and w32-
definitions here.