Add a culling slider

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Melinda had the idea that this could serve two purposes.  First, it would 
provide the user with a way to hide cells on large puzzles like the 120-cell.  
Second, if we implement it a certain way, it can be used to show cells that we 
are culling right now due to cell orientations (front-facing cells).

The proposed implementation is to have the slider change a threshold related to 
the determinants of the cell orientations.  The default of this slider would 
have a value of 0 (this point should be marked on the slider), which will give 
the current culling behavior.

Original issue reported on code.google.com by [email protected] on 24 Feb 2011 at 3:56