[FINAL] MOAIParticlePexPlugin initializing properties

I added accessor methods for two missing properties.  I added handling for minimum radius variance, since that attribute appears in Particle Designer.  It might not have been in Particle Designer when the class MOAIParticlePexPlugin was first written.  In addition, I added a method named initializeProperties() to do some of the work that load() does when setting up the registers and the read-only properties.

I verified the name of the element corresponding to minimum radius variance in a PEX file exported from a licensed copy of Particle Designer 2.0.5.4.  It is "minRadiusVariance", just like I had it in the `Parse()` method.

## TODO:

- [x] Verify the name of the element for minimum radius variance in a PEX file exported from a licensed copy of Particle Designer.