refacto: sort actions declaration, remove close from main file

fixes #921
- use view.close instead of nvim-tree.close
- put preview code inside the execution logic on keypress
- sort keys in keypress_funcs
- remove empty buffer when hijacking window

lua/nvim-tree/lib.lua

@@ -119,10 +119,6 @@ function M.open()
   end
 end
 
-function M.close_node(node)
-  require'nvim-tree.actions.movements'.parent_node(node, true)
-end
-
 function M.toggle_ignored()
   local config = require"nvim-tree.explorer.utils".config
   config.filter_ignored = not config.filter_ignored