Clearer polygon and linestring highlighting

[goldpbear]

The current spotlight-style highlighting doesn't always work well when places have non-point geometry. If a place has polygon or linestring geometry, the spotlight often doesn't cover the place's full area.

It would be great if polygon geometry were highlighted in another way, perhaps by changing a polygon's stroke to a bright color, or by fading out other layers. Maybe it's possible to add an outer glow effect by manipulating a polygon layer's underlying SVG elements.

The need for better polygon highlighting is even more apparent when using the geometry editor (introduced in #532).