When a new bridge is created, an undo command is created for that.
When undo the action and redo it, all terminals are bridged to a new
bridge instead of the first one, who continue to exist but is now
empty and 'lost' because he will never be reused.
In addition of that, if a more recent undo command (we call it undo2)
use this bridge,
there is a unknown behavior, because the status of the bridge is not the
same as when the undo2 was created.
Move RealTerminal class in a new file
Move PhysicalTerminal class in a new file.
Remove the use of QWeakPointer and use instead QSharedPointer
in a big part of the revamp.
