shell: Factor out code to set the layer for a shsurf

shell: Factor out code to set the layer for a shsurf

This will be used more extensively in the next few commits, where shsurf
layering is handled more explicitly when changing the type of a surface.

This commit introduces the minor functional change that map() will now
always add the new surface to a layer list, as
shell_surface_calculate_layer_link() always returns a non-NULL link
element. This affects fullscreen and ‘none’ surfaces (which are now added
to the fullscreen and current workspace’s layer list, respectively).