Vim keymap fixes and additions, including undo and help

Here they come:
* embarassing Home/End/Page{Up/Down} omission in Normal Vim keymap,
* added Undo to Vim Ex mode,
* added Help to Vim Ex mode,
* added <C-w>-←↑→↓ bindings to change windows and layout managers.
We can uncomment it when user scripts will be put into `yiscript.org` database like `vimscript.org`, and most users will have problem navigating through help. Not now :-).