awful.util: add table.hasitem()

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

lib/awful/util.lua.in

@@ -237,4 +237,16 @@ function table.join(...)
     return ret
 end
 
+--- Check if a table has an item and return its key.
+-- @param t The table.
+-- @param item The item to look for in values of the table.
+-- @return The key were the item is found, or nil if not found.
+function table.hasitem(t, item)
+    for k, v in pairs(t) do
+        if v == item then
+            return k
+        end
+    end
+end
+
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