Cleanse back-end code of all references to X11 types

Cleanse back-end code of all references to X11 types

When compiling for X11, common.h #included X11/Intrinsics.h so that
X11-specific data types could still be used throughout the back-end part
not shared with WinBoard. By temporarily disabling this, error messages
could be generated on all such occurrences, and were then solved. This
included the following:
  The InputSource struct is not uses in usystem.c at all, and its definition
could be moved to xboard.c. InputSourceRef, which logically should heve been
defined as (InputSource *), is in reality (void *), so the back-end can
pass it around.
  Some non-platform-dependent stuff of general interest had prototypes
(or external vars) in xboard.h. This is now moved to a new header xboard2.h,
to allow it to be shared without bringing in X11 dependence.
  The EngineOutputProc was defined inconsistently, but the header was not
sufficiently shared to flag this error. This has been repaired now, by moving
the template to menus.h.