Engine-level full-width detail row API

[bmarkov]

Master/detail works today through a sentinel-row pattern (demo 106 shows it), but you wire the expand state and the detail renderer yourself. I want to make it a real API: mark a row as having detail, hand the grid a snippet, and let it own the expand/collapse and the full-width panel.

What would you put in your detail rows - sub-grids, forms, charts?