make cascading dirs more compact

Render cascading dirs on one line i.e.

    > foo/bar/baz
      file1

instead of

    > foo
      > bar
        > baz
          > file1

This should be useful things like java projects that have deep dir
structures.

Remove the old UI view (pre the dir arrows) as this simply isnt worth
supporting for a proof of concept. This may get added back - or not.

lib/nerdtree/tree_dir_node.vim

@@ -73,6 +73,24 @@ function! s:TreeDirNode.createChild(path, inOrder)
     return newTreeNode
 endfunction
 
+"FUNCTION: TreeDirNode.displayString() {{{1
+unlet s:TreeDirNode.displayString
+function! s:TreeDirNode.displayString()
+    let vc = self.getVisibleChildren()
+    if len(vc) != 1
+        return self.path.displayString()
+    endif
+
+    let visChild = vc[0]
+
+    "TODO: optimize
+    if !visChild.path.isDirectory
+        return self.path.displayString()
+    endif
+
+    return self.path.getLastPathComponent(1) . visChild.displayString()
+endfunction
+
 "FUNCTION: TreeDirNode.findNode(path) {{{1
 "Will find one of the children (recursively) that has the given path
 "
@@ -218,6 +236,13 @@ function! s:TreeDirNode.hasVisibleChildren()
     return self.getVisibleChildCount() != 0
 endfunction
 
+"FUNCTION: TreeDirNode.isCascadable() {{{1
+"true if this dir has only one visible child - which is also a dir
+function! s:TreeDirNode.isCascadable()
+    let c = self.getVisibleChildren()
+    return len(c) == 1 && c[0].path.isDirectory
+endfunction
+
 "FUNCTION: TreeDirNode._initChildren() {{{1
 "Removes all childen from this node and re-reads them
 "