feat: tables #107

What

Tables: columns that resize, reorder, sort and hide, frozen leading columns and header rows, and row clipping so a very long table costs only its visible rows. Column state lives in the state store and persists with it. Cells hold any widget.

Part of #85. Needs hashed ids (#94) (clipped rows must not shift ids) and the state store (#95).

Acceptance

• Resize, reorder, sort, hide and freeze, with tests
• A 1,000,000 row table draws only its visible rows and a widget in a row keeps its state while rows scroll
• Column layout survives a save and load