Ticket #25711 - Added command to copy the raw apps settings from one engine to another.

To prepare for the release of the desktop application, this pull request adds a new core command to copy the apps settings from one engine to another.  The intended use is to be able to automatically setup the tk-desktop engine for a project with an existing configuration based off of the current tk-shell settings, avoiding the need to initially setup the desktop engine manually.

The new command uses a new method in Environment that copies the raw yml structure that makes up the source apps value to the destination engine.
Not really sure if this is right, but there is an option to not expose this command to the console by doing

```
self.supports_tank_command = False
```

Wondering if there is any benefit in hiding it (other than hiding it from the tank commands listing). Or am I just being overly cautious? :-)