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
83f198f1ae