Added gnome-shell-extension-tiling-assistant to the ubuntu-desktop

[Sebastien Bacher]

Hey there,

The Ubuntu Desktop team added gnome-shell-extension-tiling-assistant to 
the archive with the intend to include it in the default Ubuntu session. 
Since we expect ubuntu-desktop to maintain the package I've added it now 
to the corresponding set (which will also allow Marco who is working 
actively on the package to do the uploads himself)

Cheers,
Sébastien Bacher