container_move_to handled moving containers to new parents, as well as
moving workspaces to new outputs.
This commit removes the workspace-moving code from this function and
introduces workspace_move_to_output. Moving workspaces using
container_move_to only happened from the move command, so it's been
implemented as a static function in that file.
Simplifying container_move_to makes it easier for me to fix some issues
in #2420.
f57a3919cf