Prototype of manual editing transformation.

- Select the source you want to modify.
- Go into fuse mode.
- Press 'T'.
- Move around in the viewer. The target source does not move, while all
  the other move.
- Press 'T' again; the new transform is 'stored'.

Still plenty of bugs: the manual transform is not stored it seems.
Signed-off-by: Jean-Yves Tinevez <tinevez@pasteur.fr>