Contrary to mouse clicks, when pressing keys on a keyboard the
standard behavior is to perform the associated action immediately,
not only when releasing the key again. This should also improve the
perceived performance slightly.
Note that the 'D' shortcut had formerly been handled by Leaflet, which
we now have to do on our own.
While at it, move the character codes over to the options variable, as
found in other parts of the codebase already.
Also removing the listener from the container does not seem needed
anymore nowadays.
