rez-gui reorder packages

Since request order can influence the resolved context by virtue of package precedence, reordering packages in a request is important.
Rez-gui doesn't have a way to reorder packages short of reordering by-hand in a text editor or erasing and reconstructing the request manually.
Some up and down arrows to adjust the request order or a drag/drop mechanism would be very welcome.

Start by locating the rez-gui request editor and how it stores package order. Add a way to move packages up or down, or drag them into a new order, while preserving the order used for resolution. Done when users can reorder packages without editing or rebuilding the request and the resulting order affects precedence.

Written by the indexing model from the issue text.