Use the faster _gtk_file_system_model_get_value() function instead of
gtk_tree_model_get() inside the sort functions. This gives a significant
speed-up when sorting large lists.
In a test case with 40.000 files, the sorting time went from ~5 seconds
to less than 0.5 seconds for my test case. There is 2 significant
problems with gtk_tree_model_get() that cause this:
1) The value is copied, which takes quite a bit of time for strings.
   ~25% of excessive time or ~1 second in my test
2) The tree model functions need to lookup the interface vfunc. And
   gtk_tree_model_get() doesn't do that only once, but multiple times
   (verifying column id, getting the actual value, ...)
   ~75% of excessive time or ~3 seconds in my test