New link types that force special ways of opening the file

New link types that force special ways of opening the file

file+sys:/path/to/file     will use the system to open the file, like
                           double-clicking would
file+emacs:/path/to/file   will force opening by Emacs

Also, when using C-c C-o on a headline of get a list of links in the
entry, pressing RET will open *all* links.  This allows something like
projects to be defined, with a number of files that have to be opened
by different applications.

These were requests by John Wiegley.