Track focus history, not just currently focused item

[mjkillough]

If you:

• have multiple windows open in a group
• have the default stack layout
• have a pop-up dialog appear (like Firefox's, "are you sure you want to close these tabs?")
• dismiss the dialog

Then the original window is not always re-focused. We currently focus whichever window was most recently opened.

We should track focus history. When a focused window is closed, we should revert focus to the previously focused window.