[effects/overview] Add a little bit of padding around the window heap
This ensures that none of the windows ever touches a panel or screen edge, which is a cleaner look. It also helps for the case where there is only one maximized window open at the time the effect is activated, because with this commit, there is now a visual change to show you that something happened so you don't think than nothing happened.
I can't show the effect with a screenshot because it re-layouts itself when the Spectacle window appears/disappears. But to test the most impactful effect of this change, activate the effect when the only open window is maximized.