take &scrolloff into account when restoring the window position when
recreating the nerd tree window

store the old nerd tree window size as a tabpage local variable and
restore it when reloading the tree window

the <tab> mapping creates a new window, and we previously resized the
nerd tree window at the same time

This reverts commit 6334e99192.
Opening files silently causes vim to hang if the file is already open by
another vim instance (it tries to print out the swap file
message/options but it cant).

The quickhelp had some trailing whitespace, this was removed and a help
highlight pattern had to be changed to fit. Also, there was another
trailing space the s:RenderView() function.

previoulsy, <c-k>/<c-j> did magic stuff if the selected node was a file.
i.e. jumped to the parent/parents next sibling. I chose to remove this
behaviour as it is more confusing than helpful.

previously we were escaping when deleting files, and this was causing
the operation to fail

if the user tells the nerd tree to open a file (without splitting), and
that file is already open in the current tab, then just stick the cursor
in the existing window for the file

if the user tries to open a file and the previous window is "special"
(eg the quickfix window or another explorer plugin) then force a new
split to be opened instead

this is so that the messages get recorded in message history and can be
view with :messages

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