Add a file icon for xcf file type

This icon comes from the crystal icon set (like all the other
icons here). They are licensed LGPL. See the COPYING file
in the skins/common/images/icons directory.

It was downloaded from
https://upload.wikimedia.org/wikipedia/commons/4/4f/Xcf.png

This goes along with change I127bcb5458, which caused the icon
to be shown for indexed xcf files.

Change-Id: I72aa00c4e605ff67f62f855c1475a07433fd4567
Merge "Add a file icon for xcf file type"