Make graĥics and external insets tighter

Make graĥics and external insets tighter

Now RenderGraphics adds offsets that depends on its parent inset.

These offsets are set to 0 for InsetGraphics and InsetExternal. A nice
consequence is that icons shown by Info inset stick out less on screen.

As an unrelated change, the "private:" specifier of these two insets
is moved to a more reasonable place.