[DESIGN] Rename "Terrain" tab to "Environment" and rearrange UI, now that it includes WindLight

This needs to be discussed further, but I wanted to jot it down before I forgot. I noticed both Oz and myself had momentary confusion remembering that WindLight was in the Terrain tab. Really, some of the Terrain stuff directly affects Ground Textures. As a first rough pass, these fine steps which would resolve tension that's lasted for years:

• From Terrain tab, move the following into Ground Textures tab:
  • Water Height
  • Terrain Raise Limit
  • Terrain Lower Limit
  • Download RAW terrain
  • Upload RAW terrain
  • Bake terrain
• Rearrange the Ground Textures tab controls so it looks more pleasing, with perhaps a top "half" and a bottom "half" that distinguishes between (1) the textures that get applied to the terrain and (2) the terrain shape itself.
• Now, rename Ground Textures tab to Terrain and rename Terrain to Environment to be consistent with our usage of "environment editor", both sky and water.
• Rearrange the new Environment tab controls to make sense — details to be discussed later. Take into account changes from other issues I've reported in STORM-1126, I defer to Wolf for UI prototypes!

The result is: there's a single tab only for environment (WindLight) controls and the previously separated terrain stuff has been unified. Longer-term, I think this makes a lot of sense.