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.
Do we really need help on user context? (e.g. those defined in Env.hs, probably by the user himself)