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/flag.h

@@ -318,7 +318,6 @@ struct instance_flags {
 #ifdef TTY_SOUND_ESCCODES
     boolean vt_sounddata;    /* output console codes for sound support in TTY*/
 #endif
-    boolean clicklook;       /* allow right-clicking for look */
     boolean cmdassist;       /* provide detailed assistance for some comnds */
     boolean fireassist;      /* autowield launcher when using fire-command */
     boolean time_botl;       /* context.botl for 'time' (moves) only */