Added 'edit' check_callback
Added 'edit' check_callback

Because 'edit' and 'rename_node' differs in use, I added another
'check_callback' option where edit can be cancelled.

Right now to cancel the 'edit' command we have to block completely the
'check_callback' method, but I want to keep some 'check_callback'
options enabled and also disable the 'edit' option in some special nodes
(but not renaming programmatically).

I've done this because I don't want users to edit non-editable nodes
with F2 but I want to rename them programmatically at some point, so I
had to difference between both events.

Also, it will be less confusing as blocking the rename event will let
the user open the edit box and input something altough the node name is
not changed.

Project-TF/jstree,vakata/jstree,iCasa/jstree,iCasa/jstree,vakata/jstree,duzun/jstree,Project-TF/jstree,iCasa/jstree,duzun/jstree