[Lua] Weather API

Currently OpenMW hardcodes a small set of weathers. Each weather has associated attributes like sky texture, particle system, and fog density. These weathers and associated systems should be de-hardcoded and exposed in the Lua API.

• Change all attributes associated with a weather, like percentage, sky texture, etc...
• Read list of all registered weathers
• Add weathers
• Remove weathers
• Event to register weathers to avoid confusing interaction of multiple mods registering weathers at different points in script execution.
• Deal with special case of rain. One simple solution is to use hardcoded rendering path for rain whenever the default rain NIF is referenced.
• De-hardcode the peculiar behavior of sky rotations which look ridiculous, like blight storms close to red mountain.

Some questions:

• Will adding/removing weathers break mwscript, if so how can it be worked around?