Provide views display plugin for views-generated layers

The current implementation of the OL Views data style plugin has two problems:

1. The style plugin can be used with any normal display types, though it is actually never meant to be used with a Views display. That means if you add it to a page or block display you will have a broken view on your hands -- not good.
2. The style plugin is tied to a custom call to the pre_render() method in the OL map build process. Amongst other things, this means that the data layer cannot be cached using Views' built-in caching plugin.

This patch adds a Views display plugin for OL Views generated data layers and restricts use of the data views style plugin to this display type. The display type has none of the usual entry points (read: no paths, no blocks, etc.) so it cannot generate a broken View -- instead it only exposes OL Views data layers to OpenLayers for use in presets, etc. It also adjusts the rendering stack to go through the full Views build/execute/render process so OL data views can be cached.

Patches for 0.x, 2.x.