Consolidate XChar2b macros for the X, W32, and NS
Suggested by Jan Djärv in:
https://lists.gnu.org/archive/html/emacs-devel/2014-07/msg00203.html
* w32gui.h (XChar2b, STORE_XCHAR2B, XCHAR2B_BYTE1, XCHAR2B_BYTE2):
Move the fallback definitions to dispextern.h.
* nsgui.h (XChar2b, STORE_XCHAR2B, XCHAR2B_BYTE1, XCHAR2B_BYTE2):
Likewise. Share the same definition with W32.
* xterm.h (STORE_XCHAR2B, XCHAR2B_BYTE1, XCHAR2B_BYTE2): Move to ...
* dispextern.h (STORE_XCHAR2B, XCHAR2B_BYTE1, XCHAR2B_BYTE2):
... here for the consistency with W32 and NS.