Add check box to display selection state

With tryton we rely on the Gtk selection (and the highlight) to show what is selected. But users are less used to such design than checkboxes (thanks to the web).

So I propose to add checkboxes which display the selection state of the row similar to sao design.

Unfortunately it is very complex (and maybe impossible) to use them to actually toggle the selection because clicking on them make the treeview select the row. So I propose to insensible those checboxes so it is clear that they can not be clicked (even if user does click, the selection will be changed).