Added 'keepAlive' option that prevents PowerTip from being hidden on mouseleave events

This is a simple option that if set to true keeps the PowerTip instance visible even when the mouse leaves the trigger element (and also the tooltip content).

I don't know if this will be especially useful for other people, but in some of my use cases I am manually activating PowerTips on a click event and would like to keep them visible until they are manually closed (with another click event for example, or when a different tooltip instance is shown). I know this overlaps mouseOnToPopup a bit, in that it also provides the ability to interact with the tooltip content. But it seemed like the simplest implementation for a slightly different behavior.