0.7.6

• support display, a function translating the value a cell stores into the value it
  shows. cellcfg's format applies to whatever it returns, and editing keeps working
  on the stored value, so a cell can now hold one thing and show another.
• fix bug: the class from cellcfg and from the class option lands on the wrong cell
  in a frozen row / column once the sheet is scrolled, since the coordinate used to look
  it up treated a cell frozen on one axis as frozen on both. the idx band, which
  addresses no data of its own, was given a scrolled coordinate as well.
• fix bug: a sheet with an index column but no index row leaves the first row label
  blank, the corner test comparing both axes against the column offset.
• refactor: grid position to data coordinate now lives in one place ( _coord ) rather
  than three, and rendering a value goes through _display rather than formatting
  inline.
• fix bug: editing a cell whose rendered text differs from what it stores ( a number
  with a format from cellcfg, for one ) puts the rendered text into the editor, so
  committing writes that back as the value. 1234567 shown as 1,234,567 becomes the
  string 1,234,567, which then renders as NaN. editing now starts from the raw
  value in @_data. cells holding advanced content ( an object ) keep falling back to
  what is rendered, there being no other textual form.