MWEntityNode representing is now displayed with a light
grey background and has a tooltip explaining that this is a
non-breaking space and not a random grey blotch.
This is not done for TextNode (in core VisualEditor), as that doesn't
actually work: Parsoid converts all in input to regular spaces.
It's still not easily possible to insert a non-breaking space.
Bug: T96666
Change-Id: Icbdf7cc3e5d675b199d08777a3439dc5dedceac1
