previously it called a nonexisting function

s:ClearMarks(), s:ClearAllMarks() and s:MarkNode() now use tell the
relevant path objects to update their mark cache

This is mainly for performance reasons. Calculating the marks anew
everytime oPath#MarkNames() is called greatly increases the rendering
time of the tree. It trippled it for just a handful of marks...

create a oFileTreeNode#Refresh() and make sure that Refresh() is always
called for dir nodes

if OpenMark is called for a mark for a file that isnt cached in the
tree, just open the file anyway and dont put attempt to put the
cursor on it etc

if MakeRoot is called for a file, make sure that the nodes parent is
initialized

NERDTreeMarksFile is full path of the file that stores the marks

now if you go :MarkToRoot foo  and foo points to a path that isnt
cached, a new tree node will be created and made root. Note that all
cached nodes will be lost as this is the same as opening a new nerd tree

not that as of this commit, you can now use the C mapping on a file node
and its parent dir node will become the new root

if we were opening a mark (with :OpenMark) that was scrolled off the
screen, the tree  window wasnt getting scrolled to the node