Add an initialization script for RStation

The added script is RStation.nut.

Overlay icon fo RStation objects are added, too.
Add maxRU and defaultRU variables to RStation's initialization scirpt
Add modelFile parameter to RStation's initialization script
Delete obsolete and commented out codes from RStation
Add maxOccupyTime variable to RStation initialization script

The alphacen.nut mission demonstrates it.
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.