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.

src/options.c

@@ -6632,6 +6632,9 @@ parsebindings(char *bindings)
     uchar key;
     int i;
     boolean ret = TRUE; /* assume success */
+    static const char *const mousebtn_names[NUM_MOUSE_BUTTONS] = {
+        "mouse1", "mouse2"
+    };
 
     /* look for first comma, then decide whether it is the key being bound
        or a list element separator; if it's a key, find separator beyond it */
@@ -6660,6 +6663,17 @@ parsebindings(char *bindings)
         return FALSE; /* it's not a binding */
     *bind++ = 0;
 
+    bind = trimspaces(bind);
+
+    for (i = 0; i < SIZE(mousebtn_names); i++)
+        if (!strcmp(bindings, mousebtn_names[i])) {
+            if (!bind_mousebtn(i + 1, bind)) {
+                config_error_add("Error binding mouse button %i", i + 1);
+            } else {
+                return ret;
+            }
+        }
+
     /* read the key to be bound */
     key = txt2key(bindings);
     if (!key) {
@@ -6667,8 +6681,6 @@ parsebindings(char *bindings)
         return FALSE;
     }
 
-    bind = trimspaces(bind);
-
     /* is it a special key? */
     if (bind_specialkey(key, bind))
         return ret;