[GNC] Support message:// for transaction associations

[Ira Fuchs]

I am running Gnucash for under MacOS mojave. On MacOS it is possible to get a URL that takes you to a mail message(in mail.app). The URL is of the form <message://<3745692383.13655895.1603345168343@email.apple.com> <message://%3C3745692383.13655895.1603345168343@email.apple.com%3E>. If I enter this URL as the location for an association and then try to go to it in gnucash, I get the error message that the file cannot be found. (If the URL is pasted into Safari, it opens the email message directly). It would be very helpful if Gnucash associations could point to email invoices directly.