Allow a user-configurable delay to the 'focusFollowMouse' feature

Windows itself allows us to set a delay for its focus-follows-mouse feature, it would be nice if Terminal did too.

Ideally this would not be "additive" to Windows delay. E.g. this scenario:

• I have both delays set to 500ms
• I have two panes open in terminal, pane A and pane B
• I exit the Terminal window with pane A focussed
• I re-enter the Terminal window via pane B's area

then I should not have to wait 1000ms for pane B to gain focus, only 500ms.