Row and Column Headers, and minesweeper.html

[texodus]

Cleans up the semantics for the newly-renamed row_headers and column_headers, which are tree-like structures encoded as 2D arrays. There is lots of new documentation about how they work, and some new examples:

• virtual_indices.html shows how to configure row_headers and column_headers on a virtual data model.

• perspective_headers.html is the Perspective example, with row and column pivots enabled. The perspective data model has been updated to incorporate the removed "tree_row_header.js", for perspective-like single column tree axis rendering.

• Mega minesweeper.html is 1000x1000 Minesweeper clone written on a 2D Array data model. It can be won in ~11days if you do nothing else!

Other changes:

• Lots of vestigial perspective functionality removed, including on_select and on_sort, formatting, many metadata properties like type and value. After this cleanup is done, it may be worth revisiting the concept of metadata - it may just be easier to pass in the data model Reponse object, but with strings replaced with DOM elements that were rendered.
• In addition to formal support for row_headers and column_headers, an entirely new row_headers render mode has been added, and the render element has become a <th>. This allows much more complex row group visualizations (see the new rule marks on canvas_data_model.html), and is easier to style from CSS.
• <table> without headers can now be rendered.
• Examples updated to use new feature.
• Lots of tests.