A first try at excel-like table selection.

Click once to start selecting, click again to stop selection. Right-click on a selection for a context menu. Only option that works is "Clear selection".

Issues: when selecting, hover event only fires on table cells, so mousing over cells covered by the selection box will not fire - so you can make the selection bigger, but not smaller. This is due to the z-index of the selection box being necessarily above that of the table cells... hmmm...

Also, clicking to start the selection and then right-clicking before completing the selection by clicking again results in unexpected behavior.