Add an art seed and natural growth target for height.

1. Adds a dedicated art seed to each slave. When WebGL art is active, you can change it manually in Starting Girls or in SI Customize.  The art seed correctly duplicates onto clones and twins, and is randomized in other cases. BC copies the ID into the seed so that existing art stays the same.  Fixes #3647, obsoletes and closes !10432.
2. Sets a natural growth target for height. BC copies from existing height (minus implants) for adult slaves; a new target height is generated for child/teen slaves. Slaves that are supposed to be genetically tall or short (for example, child/teen slaves bought from neighboring arcologies with Petite Admiration or Statuesque Glorification) will now *stay* tall or short throughout their development.  Fixes #3838 properly. Also fixes #3813.
3. Adjusted physical development to directly attempt to reach the genetic target height (and in the process, dropped about 1500 lines of code).  Supports growth spurts, etc.
4. Height is now heritable, with a heritability factor of 79% as per recent peer-reviewed research (cited in comments). Activated gigantism or dwarfism will reset height inheritance.