Add "ru" (Resource Units) to Squirrel interface of RStation

Also revived property window's ru amount display.
Prevent TextureSphere from trying to load texture file name of null string

It's possible that we have a placeholder TextureSphere object with absent
texture file in prototyping phase.
Erase texture definition of Alpha Centauri's planet

The texture image is going to be prepared.
Add getStatic() method definition to RStation

It's required for omitted classname() to work properly.