Fixes ENYO-555

## Issue
If the popup has no spottable children, an [Enter] key down will cause it to be hidden because Spotlight will try to spot the nearest or last control for a 5-way key down. Since there isn't a spottable child, a control outside the popup is focused which triggers `capturedFocus` which hides the Popup.

## Fix
Remember that the key down event was [Enter] if the Popup has no spottable children and prevent the hide in `capturedFocus`.

Enyo-DCO-1.1-Signed-off-by: Ryan Duffy (ryan.duffy@lge.com)
I'm struggling to think of a scenario in which failing to do so would break something ... and yet I have no doubt someone else will find one. :) Adding a reset in `showingChanged`