On Mon, Feb 4, 2013 at 4:38 PM, Aurélien Leblond <blablack@gmail.com> wrote:

One way would be to trigger them manually (with a midi controller for example), the other is to move clips on a grid. <snip>

the idea is to be able to try out different song structure based on the same loops.

That can be done using Luppp: see following screenshot:
http://2.bp.blogspot.com/-ocw26zJzFKY/T4YskZFROrI/AAAAAAAAAQo/92cvZO6hxhI/s1600/newui.png

See the "master" section on the top right, where the words "intro", "chorus" etc are? That's the master clip launcher, it plays that "scene" on all tracks.
Load your intro loops horizontally, one per track. Hit the "master" button for that "scene" (hoziontally), and all of those loops will play in sync.
Repeat for verse, chorus, bridge, etc, and then click the buttons on the top right to experiment with order.