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.
`:param src_engine_name: xxx` style comments would be nice! (We are trying to add that everywhere as we go through and retouch the core code these days... Eventually all of it will hopefully be doc-stringed up!)