#49 reference.txt is practically unreadable. I've fixed much of it.

[Kevin Cole]

I found the formatting -- or lack thereof for reference.txt to be pretty bad. So, I created an HTML version. (I tried finding the source document from which reference.txt was extracted but was unable to find it.) I've attached the HTML version below. It could still use a bit of work, but it's an improvement -- I think -- over the original.

[Frederic Aupepin]

The purpose of the dialog is that you can double click on one of the line to insert the corresponding information in the editor.

To be noted that the content is an extract of the ABC Plus guide : https://abcplus.sourceforge.net/abcplus_en.pdf#section.A.2

Thus for now better to keep it like it is.
Link to guide are updated.