Configurable threshold - >n tabs, prompt before opening the window on click

[cxw42]

One click on a window and all of its tabs open, which can be slow if you have lots of tabs in that window. Add a configuration option n so that if a window has more than n tabs, on click, it will ask "are you sure?" before opening. n=0 => no limit. Default to 31 because it's a nice round number :) . Noted by Blank Makkara via user review 2017/09/07

[cxw42]

After #35 is fixed, we should be able to close this issue.