fixed distributed support models
add multiple seperate support models 
change default color of new support models
renamed configuration settings:
* "show_support_grid" -> "show_support_preview"
* "color_support_grid" -> "color_support_preview"


git-svn-id: https://pycam.svn.sourceforge.net/svnroot/pycam/trunk@1212 bbaffbd6-741e-11dd-a85d-61de82d9cad9