Add "type" sorting key to 'sort' option
Add empty line to sorting dialog

To visually separate keys from text label.
Extract is_directory_entry() in sort.c

Checks whether entry corresponds to a directory.
Improve formatting of sort.c:sort_dir_list() a bit
Extract opt_handlers:ensure_well_formed_sort_list

Ensures that list of sorting keys contains either "name" or "iname".
Move ensure_well_formed_sort_list to ui.c
Prepend "type" key to sort list when it's absent
Document "type" sorting key of the 'sort' option
Rename ui_view_ensure_well_formed_sort_list()

To ui_view_sort_list_ensure_well_formed().
Don't add "type" sorting key to 'sort' explicitly
Add ui.c:ui_view_sort_list_contains()

Looks for the given key in sort option.
Add "type" sorting key implicitly to 'sort'

This way it doesn't mess up view columns and sort dialog.
Merge branch 'add-type-sort-key'

Add "type" key to the 'sort' option to allow controlling grouping of
directories.

Thanks to Daniel R., a.k.a. reicheltd.

Resolves #56 on GitHub.