Provide spreadsheet-style key handling for tables

[Connor Carney]

Currently, key-handling in tables is browser-dependent. Scatterand should consistently provide spreadsheet-style handling in tables:
Enter: Move to the cell below the first cell in the current group of non-empty cells in the current row; add a row if necessary
Tab: Move cell right. Add a column if necessary.
Shift-Tab: Move cell left. At beginning of the row, move to the last cell in the previous row.
Up/Down: Move cell above/below, moving out of the table at the bottom and top