screen: add .index property

Signed-off-by: Julien Danjou <julien@danjou.info>

common/tokenize.gperf

@@ -46,6 +46,7 @@ icon_name
 id
 image
 imagebox
+index
 instance
 key
 key_press

screen.c

@@ -431,6 +431,9 @@ luaA_screen_index(lua_State *L)
 
     switch(a_tokenize(buf, len))
     {
+      case A_TK_INDEX:
+        lua_pushinteger(L, screen_array_indexof(&globalconf.screens, s) + 1);
+        break;
       case A_TK_GEOMETRY:
         luaA_pusharea(L, s->geometry);
         break;