Multiple cell selection

[lakosd]

Implement selecting a range of cells.

• [ ] Implement storing a selection of multiple cells
• [ ] Selection by click-and-drag
• [ ] Indicate starting cell when selecting with click-and-drag
• [ ] Row/column selection by clicking on row/column name
• [ ] Select all cells by clicking the first (top-left) td of the table

[lakosd]

Closed, as we broke this issue down to smaller issues