Change class names in DOM renderer

This PR changes class names in the DOM renderer.

We don't use `pane` elsewhere, so the class of the layer container is renamed from `ol-layers-pane` to `ol-layers`.

The class name of the tile layer renderer is changed from `ol-layer` to `ol-layer-tile`.

(I'm currently working on an image layer renderer, which will use `ol-layer-image` as the class name for its DOM element.)