Make file sort order and case sensitivity configurable

Set the sort order's default from the former foldersFirst to mixed, so this is a
change in behavior. I find this useful because it now matches git's order, so if
you look at the diff of a commit, the TOC at the top has the same order as the
file tree you see when entering the commit.

docs-master/Config.md

@@ -222,6 +222,13 @@ gui:
   # item at top level.
   showRootItemInFileTree: true
 
+  # How to sort files and directories in the file tree.
+  # One of: 'mixed' (default) | 'filesFirst' | 'foldersFirst'
+  fileTreeSortOrder: mixed
+
+  # If true (default), sort the file tree case-sensitively.
+  fileTreeSortCaseSensitive: true
+
   # If true, show the number of lines changed per file in the Files view
   showNumstatInFilesView: false