default config: bar position top

Long description:

Using `position top` within the bar block will prevent new users from
accidently scrolling on i3bar and changing the workspace.

To apply this patch, use:
curl http://cr.i3wm.org/patch/577/raw.patch | git am

@@ -162,6 +162,7 @@ bindsym Mod1+r mode "resize"

 # finds out, if available)

 bar {

         status_command i3status

+        position top

 }

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