table.join: Make deterministic.

`pairs` order isn't defined and `{...}` will always be ordered.
There is no reason to have random behavior where it can be
predicted at no additional cost.

lib/gears/table.lua

@@ -18,7 +18,7 @@ local gtable = {}
 -- @return A new table containing all keys from the arguments.
 function gtable.join(...)
     local ret = {}
-    for _, t in pairs({...}) do
+    for _, t in ipairs({...}) do
         if t then
             for k, v in pairs(t) do
                 if type(k) == "number" then