selection.Model: select() should only optionally set the focus

Especially in case we want to programmatically select an item, changing the focus can be problematic.

Example: we click on a next button (apps/form) which selects a new item inside the related side nav list. Moving the focus inside the side nav at this point is not the desired UX.

However: some components (e.g. gallery & helix) still want to (and should) do it.

So, a new config like focusOnSelect makes sense.

@mxmrtns