Allow binding mouse buttons

Instead of hardcoding mouse button actions, allow the user to
bind mouse buttons to extended commands.  For example the new
defaults are:

BIND=mouse1:therecmdmenu
BIND=mouse2:clicklook

Currently a bit rudimentary; the defaults should be OK, but
documentation is bit lacking, and in-game binding and option
saving are missing.

Allowed commands to bind are "nothing", "therecmdmenu", "clicklook",
and "mouseaction". Clicklook replaces the "clicklook" boolean option,
and mouseaction does what mouse 1 button used to do - a context sensitive
action.

include/wintype.h

@@ -135,6 +135,7 @@ typedef struct gi {
 /* nh_poskey() modifier types */
 #define CLICK_1 1
 #define CLICK_2 2
+#define NUM_MOUSE_BUTTONS 2
 
 /* invalid winid */
 #define WIN_ERR ((winid) -1)