Add user option to save window size/position

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](https://stackoverflow.com/users/3773011/makyen) in a [comment](https://stackoverflow.com/questions/45693634/chrome-extension-creating-window-at-the-size-of-the-default-ctrln-browser-wi?noredirect=1#comment78366866_45693634) to [this SO question](https://stackoverflow.com/q/45693634/2877364).

Start by locating the existing window size and position save/restore behavior and the user-option handling in the extension. Define how enabling and later disabling the option should treat previously saved locations, then verify both enabled and disabled flows manually in Chrome.

Written by the indexing model from the issue text.