From the original spec:

focus-pane

focus-pane [--target,-t target-pane]

Moves focus within the currently focused tab to a given pane.

Parameters:

• --target,-t target-pane: moves focus to the given target-pane. Each pane
  has a unique index (per-tab) which can be used to identify them. These
  indices are assigned in the order the panes were created. If omitted,
  defaults to the index of the currently focused pane (which is effectively a
  no-op).

This one's a bit more involved - we'll need to make sure to give every leaf pane a unique ID, and when we move a control to a new pane, make sure to move that instance's ID as well.

We might want to have a GUID->ID mapping for this. With the work over in #5000, we'll probably want to give individual terminal instances unique GUIDs, to track them across windows. Might be easiest to just track which terminal instance is in a particular pane, and then use that GUID to get the pane ID within the window.