Revert "Focus history: Don't ignore unfocusable clients"

This reverts commit c0dffca646 which caused
FS#1011. This is just too much breakage which can't easily be fixed in 3.4.

lib/awful/client.lua.in

@@ -131,10 +131,12 @@ end
 --- Update client focus history.
 -- @param c The client that has been focused.
 function focus.history.add(c)
+    if focus.filter(c) then
         -- Remove the client if its in stack
         focus.history.delete(c)
         -- Record the client has latest focused
         table.insert(data.focus, 1, c)
+    end
 end
 
 --- Get the latest focused client for a screen in history.