Convert all loading to use GFile

Now we're able to make use of GVfs, with some caveats.

fastiv.desktop

@@ -3,7 +3,7 @@ Type=Application
 Name=fastiv
 GenericName=Image Viewer
 Icon=fastiv
-Exec=fastiv -- %f
+Exec=fastiv -- %u
 Terminal=false
 StartupNotify=true
 Categories=Graphics;2DGraphics;Viewer;