In an *xref* buffer one can navigate quickly through the results with
'p' and 'n'.  However, when there is a lot of results grouped in a bunch
of files, in some cases it's more interesting to skip results and go to
the first item of the previous or next group (typically, it will be the
first result that is in a different file).  The attached patch adds two
commands for that, and binds them to 'P' and 'N', respectively.

Do you think this is a generally useful feature?  Do you think the
keybindings I chose are appropriate?  If so, feel free to install the
attached patch.  It already includes a NEWS entry and the corresponding
updates to the Emacs manual.  Thanks.