Fix SC.GridView insertionPoint position

Don't if it was working for someone but in my case, the insertionPoint was not appended.
While I was fixing it, I find out that there is no style for it and that the style for sc-list-insertion-point is only set in the legacy_theme. 

I'd love to add theme for it but I don't know where is the best place to do so: legacy_theme, ace or directly in the ressource directory by creating a grid.css.
Maybe it could also be great to move the theme for sc-list-insertion-point.

I'm waiting for work feedback before going any further.