External dep version bump: desktop-file-utils

[Vincent Untz <vuntz gnome org>]

Hi,

We currently use desktop-file-utils 0.10, which is quite old, with a few
bad bugs. I'd like to use desktop-file-utils 0.15, which is much
cleaner, better, etc. Some big parts were rewritten in version 0.13
(iirc), but since then the code is quite stable and distros are shipping
the new versions without problems.

Anyone against this change?

Vincent

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