Refactor menu code, and move it to menu.c

Refactor menu code, and move it to menu.c

Supply new method for key binding
 It is annoying that every menu item needs its X-specific handler routine,
 because this handler has to be registered to X for becoming eligible as
 a key binding. Another disadvantage of this method is that the user wanting
 to make the binding has to know the name under which the menu function
 is registered (e.g. FlipViewProc).
  A new method is now made available, which uses only a single (X-specific)
 routine, registered with X as "MenuItem", as generic key-binding handler.
 The bindings can then be made by the user specifying "MenuItem(FlipView)".
 This requires a new list of potential bindings, which should be filled
 by the menu-creation routines. (Currently only contains only a predeficed
 example.)
No longer register all MenuProcs with X
 The list with available menu procs is moved to the user-maintained
 menuItemList. Only QuitProc had to stay registered with X, because it is
 used in CatchDeleteWindow. (So it is in both lists now.) In the future,
 most of the list can be built automatically from the menu tables.
Alter type of MenuProcs to void(void)
 By getting rid of the argument list with X-types, most menu procs can
 become back-end. In the cases of ManProc and QuitProc the original
 version had to be kept as well, because in ManProc the argument was
 actually used (with key binding), while QuitProc is used for delete window.
Remove now unnecessary menu wrappers
 Now the MenuProc have an empty argument list, those that also call a
 single 'MenuEvent' without arguments have become completely redundant,
 and have been taken out of the loop by calling the corresponding MenuEvent
 directly.
Build menuItemList from menu tables
 Rather than having an initialized table of name-proc associations (which
 used to be the X registrations) for the menu items, we now build the list
 during menu cration from the menu tables. The menu "ref" is now used as
 proc name. This means the old MenuProc names used in the predefined
 key bindings had to be changed to conform to this. Note that the bindings
 would slice up names containing spaces in several parameters, so in the
 translations the spaces were squeezed out of the item names. This requires
 a hand-written comparison routine Equal() to identify the binding.
Remove prototypes of deleted MenuProcs
Move prototypes of platform-independent MenuProcs to menus.h
 Create a new header file for the menu stuff, so that we can move
 the corresponding handler routines out of xboard.c to a back-end file.
Extract back-end code from menu creation
 The menu tables and code to interpret them is moved to a new file menu.c.
 Only basic fuctions for creating a pop-down list and for adding an item
 to it remain in xboard.c.