Adds the tmuxWindowsOpenInBackground advanced setting (default off). When
enabled, new or restored tmux windows and tabs are revealed without taking
keyboard focus, so attaching with tmux -CC or having tmux create a window no
longer steals focus from the session you are working in.

The "open in background" intent is threaded as an explicit openInBackground:
parameter from loadTmuxLayout: through openTabWithTmuxLayout: down to
insertTab:atIndex:openInBackground: (with appendTab: and
addTabAtAutomaticallyDeterminedLocation: variants). Existing callers go
through thin wrappers that pass NO, so non-tmux tab insertion is unchanged.

When opening in background, insertTab: leaves the current tab selected,
reveals the window with orderWindow:NSWindowBelow: instead of
makeKeyAndOrderFront:, and skips setCurrentTerminal: so clients do not observe
a focus or window steal.

Based on the approach in PR #684 by Yifan Gao (gaoyifan), reworked to carry
the flag as an explicit parameter rather than transient window state.