Actually updating the images after setting borderColor or glowColor

Thanks for pulling so quickly, but sorry, my last pull request didn't work so well. The UIAppearance sets values only when views are added into the hierarchy and by that time the images were already generated, so borderColor and glowColor efficiently did do nothing.

With this pull request the generation of the images is delayed to right after the popover entered the hierarchy. If the popover is already visible the regeneration is done after a short delay to avoid multiple generation passes when setting multiple properties during one round of the runloop.