Fire onShow, onHide and onCancel events

[moniuch]

Please add @output emitters onShow, onHide and onCancel (onCancel would be emitted on outside click and always trigger onHide alongside), so that consumers can control other UI elements accordingly. Especially onHide/onCancel would serve well the use case where you want to toggle row highlight when the popover is used to display row-related content.