Add user option to save window size/position

[cxw42]

I prefer not saving window size/position since I may use the same set of tabs for different tasks with different window layouts. However, another option is to save size/position. Add an option to do so, and sensible handling if the user enables and later disables the option. If the option is disabled, should previously-saved windows re-open in their saved locations anyway?

Suggested by Mayken in a comment to this SO question.

[erikrob]

And in addition to position and size, I would recommend adding monitor (for multi monitor configs) and space / virtual desktop.
I'm a multi monitor user and use osx spaces on each monitor. So on every compute restart I end up opening my 7 usual browser windows via tabfern and moving them manually to the right monitor and space every single time. I'd love to have this done automatically for me, such a time saver !

[cxw42]

@erikrob thanks for using TF; makes sense to me! PRs welcome :) .

[cxw42]

Related: #182, #222 (new windows, not existing windows, but some of the code might be applicable here).