TODOs: 

* Allow the definition of lines outside of cells.
  Currently lines are rendered by CellRenderers. The table assumes a
  1px border, but this must be drawn by cellrenderers.
  
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PARTLY DONE:
* Profile for performance on large tables. 
  State: Results are acceptable, but small tweaks are possible.
  Look at the huge examples (BooleanModelExample and SpanModelExample)
  that have >=100000 rows and cols.
