Initial implementation of keysym handlers

Initial implementation of keysym handlers

Add the xkbcommon implementations of XKeysymToString and XStringToKeysym.
These symbols have the namespace prefix of Xkbc and are declared in
X11/XkbCommon.h.

The implementation is taken directly from Xlib, but does not include the
XKeysymDB parsing and hashing yet (if it ever will). A couple type
conversions were needed to keep from using Xlib.h. See original files:

libX11/src/KeysymStr.c
libX11/src/StrKeysym.c