refactor out the code that actually opens the explorer window into
s:OpenExplorerFor(treenode) so that we can call it separately

if a bookmark is created, remove any previously cached bookmark of the
same name from the tree

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