W.Popover - better display for flex containers

[georgesboris]

When dealing with flex containers (flex-row + items-stretch) we're getting some problems with how popover ends up locking the trigger element out of the base layout. We should study some way to make W.Popover to be as flexible as possible for different use cases, since the ideal scenario would be for it to be layout-agnostic (in the future this should be solved by html's popover element).