nokia-n900: Set tabbed workspace layout as default for i3

For i3 on the N900, which seems to be the only device using i3 at the moment, I thought using Tabbed workspace layout as the default would be best, since the screen is quite small, and having two split windows can quickly make it impractical.

With this commit, all new windows will be created in tabbed configuration. It is still possible to switch to stacking or standard layout as needed, using the appropriate shortcut keys.