DataTable improvements (and more)

[darrenburns]

Added

• Different cursor types: cell (pre-existing), row, column, none. If you want no cursor at all, you can use the "none" cursor.
  • Scrolling (with arrow keys) perpendicular to the cursor type will move the cursor.
  • Scrolling (with arrow keys) parallel to the cursor type will scroll as normal without shifting the cursor.
  • When "none" cursor is used, arrow keys are used just for scrolling.
• Ability to switch cursor off table.show_cursor = False. This will hide the cursor. The cursor state is retained when re-enabled. When you disable it, scrolling with arrow keys works as normal. When you re-enable it, the cursor will be scrolled to.
• Hover cursor will now only become visible when you interact with the table using the mouse. It'll hide again when you use the keyboard.
• Events for row/col/cell highlighting and selection.
• Improving reactive tests - some of them had redundant code or things commented out that were previously broken.
• Extract Coord class into own module for reuse, and rename it to Coordinate.

datatable-cursors-16jan23.mov

Issues encountered

• show_cursor was part of the API but not actually implemented.
• When fixed columns are used, the cursor isn't visible until you move it.
• Component style for datatable--even-row doesn't apply at all (but odd works).
• Crash if number of fixed_columns is equal to number of columns (presumably when greater too).
• (Possibly missing API) We need to clamp cursor position when number of fixed_columns changes, so we clamp and then assign it. Since we assign it, it triggers the validator which clamps it again, repeating the work.
• Vertical scrollbar doesn't update when show_header set to False.

Please review the following checklist.

• Docstrings on all new or modified functions / classes
• Updated documentation
• Updated CHANGELOG.md (where appropriate)

[darrenburns]

We'll probably need to change the names of these when we do "labelled" rows/columns - the fixed/header concepts are a little conflated at the moment.

[darrenburns]

always_update is required as sometimes we need to force validation via self.cursor_cell = self.cursor_cell.

[willmcgugan]

LGTM

We should think about an example that uses the data-table. One for another PR.