Rewrite most of grab_menu in grab.c (it was partly 9wm code).

This should work functionally the same, with a few simplifications.

Changes:
- we don't care if you're holding another button when you release the
  menu key if you don't want to select anything, move off the menu.
- remove the hysteresis from the menu selection (before you had to move
  more than three pixels onto a new menu entry before it selected it)
- simplify a lot of the selection code
- kill dead code.
- do what the XXX comment said and cache the screensize (i may tweak
  this later).

As far as I can tell, the only code remaining from 9wm is the list of
fonts in calmwm.c. Others appear to concur.

ok marc@, looked over and tested by a few others. Reminders from okan@.

calmwm.h

@@ -79,6 +79,10 @@ struct screen_ctx {
 
 	int		 altpersist;
 
+	int		 maxinitialised;
+	int		 xmax;
+	int		 ymax;
+
         FILE            *notifier;
 
 	struct cycle_entry_q mruq;
@@ -276,7 +280,6 @@ struct menu {
 	char  text[MENU_MAXENTRY + 1];
 	char  print[MENU_MAXENTRY + 1];
 	void *ctx;
-	short lasthit;
 	short dummy;
 };
 
@@ -399,7 +402,6 @@ int   grab_drag(struct client_ctx *);
 void  grab_menuinit(struct screen_ctx *);
 void *grab_menu(XButtonEvent *, struct menu_q *);
 void  grab_label(struct client_ctx *);
-void  grab_exec(void);
 
 void  xfree(void *);
 void *xmalloc(size_t);