Introduce a new struct ViewDir to handle directory views common

Introduce a new struct ViewDir to handle directory views common
data.
Specific data is now in ViewDirInfoList and ViewDirInfoTree.
Type of directory view can be specified with enum DirViewType.
This is saved to rc file as layout.dir_view_type, which replace
layout.view_as_tree.
Code was modified to reflect these changes.
This is a first to move to merge common code of view_dir_list.c
and view_dir_tree.c and ease the introduction of new types
of directory view.