CHUI-362 ADD. FIX (When the "Bring back" panel is there, it is not possible to...

CHUI-362 ADD. FIX (When the "Bring back" panel is there, it is not possible to collapse the right side of the conversation dialog: the toolbar is completely missing and the "<<" chevrons are not there): This was because the button "<<" belonged session's floater and not the container. Therefore, when all the floaters in the stack becomes invisible, their buttons also become invisible. Solution: Added additional button "<<" in the stub_panel of the container.