Basic Modifiers

So this is a fairly massive chunk. I tried to get our Plasma Modifiers to
match the Blender Modifier UI fairly well. In the process, I discovered
that Blender "helpfully" hides the modifier button on Empty objects. Bah.

Significant Changes:
- Hid all of the Blender Physics mess
- The Physics context is now the Plasma Object context
- Moved Plasma Object and Plasma Synchronization to the Context Formerly
  Known as Physics
- Added a Plasma Modifier Panel

Here's how you create Plasma Modifiers:
- Add your PropertyGroup to properties.modifiers
- Make sure you have a pl_id naming your modifier, a category, and a label
- Implement the export() method to actually export something useful (not
  goat porn, please)
- Implement your UI draw function in ui.modifiers.
- Wasn't that easy?