This adds touchscreen support in the diff viewer for selecting rows and

creating comments, and for opening existing comments. When using a

device like an iPad, users can now tap-and-drag down line numbers to

create a new comment, or tap a line with an existing comment to view it.

This works just like using a mouse.
The page will not scroll when creating a selection, helping to keep

selections predictable. Dragging in any other column (such as the code

columns) will continue to scroll the page.
Selections can be cancelled by ending a drag outside of the line numbers

column.

Tested creating single-line comments on an iPad.
Tested creating multi-line comments on an iPad.
Tested dragging upwards and downwards, ensuring the selection is always

correct.
Tested ending a selection outside of the line numbers column to cancel

the selection.
Tested tapping a line with an existing comment to view that comment.
Tested scrolling the page by dragging in other parts of the page.
Tested that all mouse operations still worked correctly.