dia.desktop does not list all supported MIME types

[Jason Eisner]

Dia is able to read SVG files and also save them (via export). However, the GNOME desktop file /usr/share/applications/dia.desktop does not advertise this fact. It lists only
    MimeType=application/x-dia-diagram;

(This has consequences e.g. for the MyPaint application, which allows use of an external program to edit vector graphics. It does not list Dia as an option for editing SVG.)

The MimeType line should *at least* contain
    MimeType=application/x-dia-diagram;image/svg+xml;
(I have confirmed that this change fixes the MyPaint problem after running update-desktop-database.)

However, I am sure that there are other MIME types that Dia can handle, and these should be listed as well in dia.desktop.

I am using Linux Mint 17.1 Rebecca, MATE edition, based on Ubuntu 14.04.
I am using the dia-common package version 0.97.2-15ubuntu1.