"Keybindings" window should point to docs

[bennn]

The Keybindings window should link to the documentation for keymap strings, so a new user can figure out what c:~m:\ means.

The relevant docs are on the page for keymap% (thanks @rfindler ) http://docs.racket-lang.org/gui/keymap_.html#%28meth._%28%28%28lib._mred%2Fmain..rkt%29._keymap~25%29._map-function%29%29

I think the easiest fix is to add a button that says "Help" hyperlinked to the URL above.

[bennn]

Here's a picture of the "Keybindings" box on my machine.

[rfindler]

Probably better to link to DrRacket documentation?