Getting point-and-click working

[David Sumbler]

I put off trying to get point-and-click for several years (!) until
now, because the instructions in the Usage Manual seemed rather
cryptic.  But point-and-click would be massively useful, so I thought I
would try to set it up.  I followed the instructions to the best of my
limited ability and, rather as I expected, it didn't work for me.

I am running Ubuntu 18.04 with emacs 25.2. and the Unity desktop.

I have LYEDITOR set to "emacsclient --no-wait +%(line)s:%(column)s %" file)s".
I don't have EDITOR set to anything at the moment.

I couldn't seem to find whether I was running gnome 2 or 3, so I
carried out the steps documented for gnome 2, but running

        gnome-open textedit:///etc/issue:1:0:0

produced an error saying that the URL could not be found.

So I tried the gnome 3 steps.  I created a file 
        ~/tmp/lilypond-invoke-editor.desktop
containing the material specified in the UM, i.e.:

        [Desktop Entry]
        Version=1.0
        Name=lilypond-invoke-editor
        GenericName=Textedit URI handler
        Comment=URI handler for textedit:
        Exec=lilypond-invoke-editor %u
        Terminal=false
        Type=Application
        MimeType=x-scheme-handler/textedit;
        Categories=Editor
        NoDisplay=true

I then executed:

        xdg-desktop-menu install ./lilypond-invoke-editor.desktop
        xdg-mime default lilypond-invoke-editor.desktop 
x-scheme-handler/textedit

Now when I start emacs, run the command server-start and then issue the
command

        gnome-open textedit:///etc/issue:1:0:0

the file /etc/issue does indeed open in emacs.  It contains the single
line

        Ubuntu 18.04.1 LTS \n \l

This seemed promising, to me.

The PDF viewer I use is evince, so I added the lines

        # For Textedit links
        /usr/local/bin/lilypond-invoke-editor Cx -> sanitized_helper,

to the file /etc/apparmor.d/local/usr.bin.evince (which was previously
empty), and then ran

        sudo apparmor_parser -r -T -W /etc/apparmor.d/usr.bin.evince

as instructed in the UM.

However, when I open a lilypond PDF file in evince and click on a note,
I get an error message: 

        Unable to open external link.
        Failed to execute child process “lilypond-invoke-editor” (Permission 
denied)

Yet I can run lilypond-invoke-editor from a command line without any
parameters, and I get the expected usage help.

Can anyone suggest what I have done wrong or omitted to do?

David