Add support for opening Xcursor files

Sadly, they don't have a canonical extension, and they don't show up
in the browser.  We might want to employ some level of sniffing.
The first 16 bytes are enough to identify a lot.

fastiv.desktop

@@ -8,4 +8,4 @@ Terminal=false
 StartupNotify=true
 Categories=Graphics;2DGraphics;Viewer;
 # TODO(p): Generate this list from source files.
-MimeType=image/png;image/bmp;image/gif;image/jpeg;image/x-dcraw;image/svg+xml;
+MimeType=image/png;image/bmp;image/gif;image/jpeg;image/x-dcraw;image/svg+xml;image/x-xcursor;